docs: Rename `user.md` to `helpcenter.md`.

Renames `/docs/documentation/user.md` to reflect the rebranding
from "user documentation" to "help center documentation".

Also, fixes any linking in the docs to that file.

docs/documentation/api.md

@@ -30,7 +30,7 @@ Our API documentation is defined by a few sets of files:
   [OpenAPI description](../documentation/openapi.md) at
   `zerver/openapi/zulip.yaml`.
 - The documentation is written the same Markdown framework that powers
-  our [user docs](../documentation/user.md), with some special
+  our [help center docs](../documentation/helpcenter.md), with some special
   extensions for rendering nice code blocks and example
   responses. Most API endpoints share a common template,
   `templates/zerver/api/api-doc-template.md`, which renders the

docs/documentation/index.md

@@ -6,7 +6,7 @@ maxdepth: 3
 ---
 
 overview
-user
+helpcenter
 integrations
 api
 openapi

docs/documentation/overview.md

@@ -103,7 +103,7 @@ designed to explain how the product works to end users. We aim for
 this to be clear, concise, correct, and readable to nontechnical
 audiences where possible.
 
-See our guide on [writing help center articles](user.md).
+See our guide on [writing help center articles](helpcenter.md).
 
 ### Integrations documentation
 

docs/tutorials/new-feature-tutorial.md

@@ -158,7 +158,7 @@ For detailed information on the kinds of documentation Zulip has, see
 
 **Help center documentation:** You will likely need to at least update,
 extend and link to `/help/` articles that are related to your new
-feature. [Writing help center articles](../documentation/user.md)
+feature. [Writing help center articles](../documentation/helpcenter.md)
 provides more detailed information about writing and editing feature
 `/help/` articles.
 
@@ -695,7 +695,7 @@ file documenting the feature to `templates/zerver/help/` in the main
 Zulip server repository, where the source for Zulip's end user
 documentation is stored. Details about writing, editing and testing
 these Markdown files can be found in:
-[Writing help center articles](../documentation/user.md).
+[Writing help center articles](../documentation/helpcenter.md).
 
 Also, new features will often impact Zulip's REST API documentation,
 which is found in `zerver/openapi/zulip.yaml`. You may have noticed