docs: Update writing documentation for starlight help center.

The help center documentation now uses astro/starlight, so the
contributor documentation is updated here for the general changes
to that documentation, such as where to find files, and features
and components used in that documentation.

Moves features that are still used for integrations and API docs,
to those guides, e.g., tabbed sections and our macros system, so
that we still have documentation for those systems/features.

Co-authored-by: Shubham Padia <shubham@zulip.com>

docs/documentation/overview.md

@@ -12,11 +12,11 @@ Zulip has three major documentation systems:
   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.
 
-- **User-facing 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 have
-  several hundred pages written using this system. There are 3
-  branches of this documentation:
+- **User-facing documentation**: Zulip uses a scalable system for
+  documenting Zulip's integrations and REST API, without a lot of
+  overhead or duplicated code/syntax, written in Markdown. Zulip's
+  help center uses [@astro/starlight](https://starlight.astro.build/),
+  with most of the content written in MDX.
   - [Help center documentation](#help-center-documentation)
     (with a target audience of individual Zulip users)
   - [Integrations documentation](#integrations-documentation)
@@ -90,11 +90,16 @@ important elements of the product clearly.
 
 ## User-facing documentation
 
-All of these systems use a common Markdown-based framework with
-various extensions for macros and variable interpolation,
+Zulip's API and integrations documentation use a common Markdown-based
+framework with various extensions for macros and variable interpolation,
 (`render_markdown_path` in the code), designed to make it convenient
 to do the things one does a lot in each type of documentation.
 
+Zulip's help center is built with [@astro/starlight](https://starlight.astro.build/).
+Starlight is a full-featured documentation theme built on top of the
+[Astro](https://astro.build/) framework. Astro is a web framework designed
+for content driven websites.
+
 ### Help center documentation
 
 Zulip's [help center](https://zulip.com/help/) documentation is