Add some docs - lots of outdated part.

CONTRIBUTING.md

@@ -4,28 +4,20 @@
 
 * [Contributing code to Matrix](#contributing-code-to-matrix)
 * [Android Studio settings](#android-studio-settings)
-  * [Template](#template)
 * [Compilation](#compilation)
 * [I want to help translating Element](#i-want-to-help-translating-element)
 * [I want to submit a PR to fix an issue](#i-want-to-submit-a-pr-to-fix-an-issue)
   * [Kotlin](#kotlin)
   * [Changelog](#changelog)
   * [Code quality](#code-quality)
-    * [Internal tool](#internal-tool)
     * [ktlint](#ktlint)
     * [knit](#knit)
     * [lint](#lint)
   * [Unit tests](#unit-tests)
   * [Tests](#tests)
   * [Internationalisation](#internationalisation)
-    * [Adding new string](#adding-new-string)
-      * [Plurals](#plurals)
-    * [Editing existing strings](#editing-existing-strings)
-    * [Removing existing strings](#removing-existing-strings)
-    * [Renaming string ids](#renaming-string-ids)
-    * [Reordering strings](#reordering-strings)
   * [Accessibility](#accessibility)
-  * [Layout](#layout)
+  * [Jetpack Compose](#jetpack-compose)
   * [Authors](#authors)
 * [Thanks](#thanks)