[Meta] Update RFC Process

[natemoo-re]

Revamps our proposal process to turn this repository into the Astro Project Roadmap.

• Glossary
• Stage 1: Proposal
• Stage 2: Accepted Proposal
• Stage 3: RFC \& Development
• Stage 4: Ship it!

[natemoo-re]

Docs involvement?

• Different audience
• Not explaining why API was chosen, explaining how API works
  • Need a good argument for the change and also good explanation of the change

[FredKSchott]

Sad I missed the RFC call where this was discussed! I'd just call out that all of these templates are mostly copy-pasted from the current RFC template contents. Since this PR is more focused on the overall process, I'd personally consider any serious/deeper changes to the template contents/body as not directly related to this PR and something that could be tackled separately.

If we can keep this PR more focused on the process, then a follow-up PR could focus entirely on improving the actual RFC template itself, and probably benefit from the better focus.

[natemoo-re]

Decided to tackle the specific feedback around the exact templates after merging.