Clean up the community membership & contributing docs

[jtraglia]

PR Description

• Remove reference to nonexistent /community/code-reviews.md document.
• Remove duplicate "Pull Request Labels" header.
• Remove ToC entry for "Git Commit Messages & Pull Request Messages" section.
• Rename "Code Review" section to "Code Reviews" (plural) to match table of contents.
• Use correct numbers for list (I know it renders as the same).
• Format membership responsibilities table.
• For ONLY the community membership document, line wrap to 80 characters.
  • I noticed that some paragraphs were wrapped and some weren't.
  • Went with 80 instead of 120 because that's what other paragraphs were at.
• To maintain consistency, add blank line between header and paragraphs in some places.
• To maintain consistency, remove periods from list entries.
• To maintain consistency, use only one space between new sentences.

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[siladu]

Looks good, just one minor thing

[siladu]

LGTM