docs: Extend our documentation of our documentation.

The main purpose of this effort is to document our new /help/ docs,
but we update a bunch of other stuff while we're at it.

docs/README.md

@@ -1,13 +1,39 @@
 # Documentation
 
-These docs are written in [Commonmark
-Markdown](http://commonmark.org/) with a small bit of rST.  We've
-chosen Markdown because it is [easy to
-write](http://commonmark.org/help).  The docs are served in production
-at [zulip.readthedocs.io](https://zulip.readthedocs.io/en/latest/).
+Zulip has three major documentation systems:
 
-If you want to build the documentation locally (e.g. to test your
-changes), the dependencies are automatically installed as part of
+* Developer and sysadmin documentation: Documentation for people
+  actually interacting with the Zulip codebase (either by developing
+  it or installing it), and written in Markdown.
+
+* Core website documentation: Complete webpages for complex topics,
+  written in HTML, JavaScript, and CSS (using the Django templating
+  system).  These roughly correspond to the documentation someone
+  might look at when deciding whether to use Zulip.  We don't expect
+  to ever have more than about 10 pages written using this system.
+
+* General user documentation: Our scalable system for documenting
+  Zulip's huge collection of specific features without a lot of
+  overhead or duplicated code/syntax, written in Markdown.  We expect
+  to eventually have around 100 pages written using this system.  The
+  target audience for this system is individual Zulip users.
+
+These three systems are documented in detail below.
+
+## Developer and sysadmin documentation
+
+What you are reading right now is part of the collection of
+documentation targeted at developers and people running their own
+Zulip servers.  These docs are written in
+[Commonmark Markdown](http://commonmark.org/) with a small bit of rST.
+We've chosen Markdown because it is
+[easy to write](http://commonmark.org/help).  The source for Zulip's
+developer documentation is at `docs/` in the Zulip git repository, and
+they are served in production at
+[zulip.readthedocs.io](https://zulip.readthedocs.io/en/latest/).
+
+If you want to build the developer documentation locally (e.g. to test
+your changes), the dependencies are automatically installed as part of
 Zulip development environment provisioning, and you can build the
 documentation using:
 
@@ -20,15 +46,75 @@ and then opening `file:///path/to/zulip/docs/_build/html/index.html` in
 your browser (you can also use e.g. `firefox
 docs/_build/html/index.html` from the root of your Zulip checkout).
 
-If you are introducing a new section into the table of contents,
-you will want to modify `docs/index.rst` and run `make clean` before
-`make html`, so that other docs besides your new one also get the
-new entry in the table of contents.
+If you are adding a new page to the table of contents, you will want
+to modify `docs/index.rst` and run `make clean` before `make html`, so
+that other docs besides your new one also get the new entry in the
+table of contents.
 
 You can also usually test your changes by pushing a branch to GitHub
 and looking at the content on the GitHub web UI, since GitHub renders
-Markdown.
+Markdown, though that won't be as faithful as the `make html`
+approach.
 
 When editing dependencies for the Zulip documentation, you should edit
 `requirements/docs.txt` (which is used by ReadTheDocs to build the
-documentation quickly, without installing all of Zulip's dependencies).
+Zulip developer documentation, without installing all of Zulip's
+dependencies).
+
+## Core website documentation
+
+Zulip has around 10 HTML documentation pages under `templates/zerver`
+for specific major topics, like the features list, client apps,
+integrations, hotkeys, API bindings, etc.  These documents often have
+somewhat complex HTML and JavaScript, without a great deal of common
+pattern between them other than inheriting from the `portico.html`
+template.  We generally avoid adding new pages to this collection
+unless there's a good reason, but we don't intend to migrate them,
+either, since this system gives us the flexibility to express these
+important elements of the product clearly.
+
+## General user documentation
+
+Our goal is for Zulip to have complete, high-quality user-facing
+documentation about how to use every feature and how to do common
+tasks (like setting up a new Zulip organization well).  This system is
+designed to make writing and maintaining such documentation highly
+efficient.
+
+The user documentation is available under `/help/` on any Zulip
+server;
+(e.g. [https://chat.zulip.org/help/](https://chat.zulip.org/help/) or
+`http://localhost:9991/help/` in the Zulip development environment).
+The user documentation is not hosted on ReadTheDocs, since Zulip
+supports running a server completely disconnected from the Internet,
+and we'd like the documentation to be available in that environment.
+
+The source for this user documentation is the Markdown files under
+`templates/zerver/help/` in the
+[main Zulip server repository](https://github.com/zulip/zulip).  The
+file `foo.md` is automatically rendered by the `render_markdown_path`
+function in `zerver/templatetags/app_filters.py` when the user
+accesses a URL of the form `/help/foo`; with special cases for
+`/help/` going to `index.md` and `/help/unknown_article` going to
+`missing.md` (with a 404 response).  Images are usually linked from
+`static/images/help/`.
+
+This means that you can contribute to the Zulip user documentation by
+just adding to or editing the collection of markdown files under
+`templates/zerver/help`.  If you have the Zulip development
+environment setup, you simply need to reload your browser on
+`http://localhost:9991/help/foo` to see the latest version of `foo.md`
+rendered.
+
+Since raw HTML is supported in Markdown, you can include arbitraty
+HTML in your documentation in order to do fancy things like
+highlighting an important aspect of your code.  We'll likely add a
+library of common components over time, which will be documented
+below.
+
+### Supported features
+
+* All the usual features of Markdown with raw HTML enabled so you can
+  do custom things as needed.
+* Code blocks with syntax highlighting.
+* Anchor tags for linking to headers in other documents.