docs: Add sidebar title collapse functionality and design facelift (#43754)

This PR adds the ability to collapse section in the docs sidebar (which
are persistent until you close the tab), and some design facelift to the
docs, which makes its design close to the site as well as polishing up
many elements and interactions (like moving the search to a modal and
making the table of content visible in smaller breakpoints).

<img width="600" height="2270" alt="Screenshot 2025-11-28 at 5  26@2x"
src="https://github.com/user-attachments/assets/3a8606c6-f74f-4bd2-84c8-d7a67ff97564"
/>

Release Notes:

- N/A

docs/src/SUMMARY.md

@@ -1,6 +1,6 @@
 # Summary
 
-# Getting Started
+# Welcome
 
 - [Getting Started](./getting-started.md)
 - [Installation](./installation.md)

docs/src/all-actions.md

@@ -1,3 +1,3 @@
-## All Actions
+# All Actions
 
 {#ACTIONS_TABLE#}

docs/src/configuring-languages.md

@@ -1,4 +1,4 @@
-# Configuring supported languages
+# Configuring Supported Languages
 
 Zed offers powerful customization options for each programming language it supports. This guide will walk you through the various ways you can tailor your coding experience to your preferences and project requirements.