docs: Truncate snippet-related info

Description

Removes the snippets-related info from the README that is being ported into the gitlab repo in gitlab!169195 (merged).

• Point overview link to the GitLab docs
• Add the missing command, and match current style. It's not well- formatted, but fixing it here is scope creep.
• Delete the subheadings.
• In a separate commit (in case I need to reverse it) deletes these images:
  • docs/assets/create-snippet.gif (172 KB)
  • docs/assets/insert-multi-file-snippet.gif (871 KB)
  • docs/assets/snippet-patch.mp4 (880 KB)

Related Issues

• Related to Plan for migrating VS Code user documentation (#1530)
• Depends on Add snippet-related items to VS Code page (gitlab!169195 - merged) being merged first into the gitlab repo

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