docs: Rename docs/README.html to docs/documentation.html.

Since it's now the top-level documentation for our documentation
system in general, not just docs/, it deserves a more normal name.

docs/index.rst

@@ -157,5 +157,5 @@ Contents:
    release-checklist
    api-release-checklist
    swagger-api-docs
-   README
+   documentation
    user-docs

docs/new-feature-tutorial.md

@@ -125,7 +125,7 @@ information on writing and running tests, see the
 After implementing the new feature, you should
 document it and update any existing documentation that might be
 relevant to the new feature. For more information on the kinds of
-documentation Zulip has, see [Documentation](README.html).
+documentation Zulip has, see [Documentation](documentation.html).
 
 ## Example Feature