Automated RFC workflow

Ideally, submitting a proposal should be as simple as writing the markdown document and clicking a button. The current workflow is a little too involved for non-techy users or even just people unfamiliar with the process here. It leaves too much room for potential mess such as:

- Incorrectly submitted RFCs (e.g. to the wrong directory)
- Inconsistent filenames and titles
- Lack of or mis-linked discussions and fully rendered proposal links
- People discussing the proposal in PR comments instead of in the discussion

Ultimately, I think we'd want to create a simple frontend for people submitting, editing and viewing proposal documents, but it would still use this repo behind the scenes. Devs would continue to use github for reviewing, discussing and managing RFCs.

The issue names no files, tests, or entry points; start by documenting the current RFC submission, review, and discussion process in this repository. Define the frontend scope and repository workflow with maintainers first; done should be an agreed, actionable design or implementation plan that addresses the listed submission problems.

Written by the indexing model from the issue text.