docs: Cut and reshuffle features section

Description

Cuts more information out of this page. Because I have a merge request waiting ahead of this one, I've set this MR to merge into 1530-cut-account-stuff instead of main. What this MR does:

• Cuts these subheadings, because gitlab!173203 (merged) moves them into the gitlab repo:
  • Set token with environment variables
  • Merge Request Reviews
  • Compare with default branch
  • Open active file
• Deletes images:
  • docs/assets/compare-with-master.gif (261 KB)
  • docs/assets/diff-comments.gif (5.3 MB)
  • docs/assets/open_active_file.gif (1.2 MB)
• Takes the big list of features from the main subheading, and moves it to under Features. I then split the list apart into a few NEW sub-sections of Features, to help users grasp the feature set quickly:
  • Manage projects and code
  • Manage issues and merge requests
  • AI-assisted features
  • Other features

Related Issues

• Depends on Adds more subheadings to VS Code page (gitlab!173203 - merged)
• Merges into docs: Remove sections about account management (!2148 - merged) because that work hasn't merged yet
• Related to Plan for migrating VS Code user documentation (#1530 - closed) which tracks the overarching work

How has this been tested?

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes