docs: Fix a lot of redirecting / broken links

Description

Fixes broken or redirecting links in the documentation. I have a couple that I don't know the answers for; I'll add comments to those spots.

Related Issues

Related to Run link-checker against all repos, all docs (gitlab-org/editor-extensions/meta#103 - closed)

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