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/integrations.md

@@ -253,3 +253,99 @@ to find. A few things to keep in mind:
   the text is referring to that is difficult to locate.
 - Screenshots should never include the entirety of a third-party website’s page.
 - Each screenshot should come after the step that refers to it.
+
+## Markdown features
+
+Zulip's Markdown processor allows you to include several special features in
+your documentation to help improve its readability:
+
+- Since raw HTML is supported in Markdown, you can include arbitrary
+  HTML/CSS in your documentation as needed.
+
+- Code blocks allow you to highlight syntax, similar to
+  [Zulip's own Markdown](https://zulip.com/help/format-your-message-using-markdown).
+
+- Anchor tags can be used to link to headers in other documents.
+
+- Inline [icons](#icons) are used to refer to features in the Zulip UI.
+
+- Utilize [macros](#markdown-macros) to limit repeated content in the documentation.
+
+- Create special highlight warning blocks using
+  [tips and warnings](#tips-and-warnings).
+
+- Format instructions with tabs using
+  [Markdown tab switcher](#tab-switcher).
+
+### Icons
+
+See [icons documentation](../subsystems/icons.md). Icons should always be
+referred to with their in-app tooltip or a brief action name, _not_ the
+name of the icon in the code.
+
+### Tips and warnings
+
+A **tip** is any suggestion for the user that is not part of the main set of
+instructions. For instance, it may address a common problem users may
+encounter while following the instructions, or point to an option for power
+users.
+
+```md
+!!! tip ""
+
+    If you want notifications for issues, as well as events, you can
+    scroll down to **Webhooks** on the same page, and toggle the
+    **issue** checkbox.
+```
+
+A **keyboard tip** is a note for users to let them know that the same action
+can also be accomplished via a [keyboard shortcut](https://zulip.com/help/keyboard-shortcuts).
+
+```md
+!!! keyboard_tip ""
+
+    Use <kbd>D</kbd> to bring up your list of drafts.
+```
+
+A **warning** is a note on what happens when there is some kind of problem.
+Tips are more common than warnings.
+
+```md
+!!! warn ""
+
+    **Note**: Zulip also supports configuring this integration as a
+    webhook in Sentry.
+```
+
+All tips/warnings should appear inside tip/warning blocks. There
+should be only one tip/warning inside each block, and they usually
+should be formatted as a continuation of a numbered step.
+
+### Tab switcher
+
+Our Markdown processor supports easily creating a tab switcher widget
+design to easily show the instructions for different languages in API
+docs, etc. To create a tab switcher, write:
+
+```md
+{start_tabs}
+{tab|python}
+# First tab's content
+{tab|js}
+# Second tab's content
+{tab|curl}
+# Third tab's content
+{end_tabs}
+```
+
+The tab identifiers (e.g., `python` above) and their mappings to
+the tabs' labels are declared in
+[zerver/lib/markdown/tabbed_sections.py][tabbed-sections-code].
+
+This widget can also be used just to create a nice box around a set of
+instructions
+([example](https://zulip.com/help/deactivate-your-account)) by
+only declaring a single tab, which is often used for the main set of
+instructions for setting up an integration.
+
+[tabbed-sections-code]: https://github.com/zulip/zulip/blob/main/zerver/lib/markdown/tabbed_sections.py