add: SIMD linting

[jacobcreech]

Using Unboxed Software's POC in #36 I updated to be added as a part of the CI process.

CI will now fail the PR if the following criteria are not met:

• 80 character line wrap
• Must have expected headings (Summary, Motivation, Alternatives Considered, New Terminology, Detailed Design, Impact, Security Considerations)
• Must have proposal frontmatter expected headings (simd, title, authors, category, type, status, created)
• Must have valid proposal type and category

and a few other markdown linting additions.

[jacobcreech]

Pulling in. We can update if they're too restrictive