* Update the RFC process documentation

- Nudge authors towards using Semantic Line Breaks for writing the RFC text
- Nudge authors towards posting their text as pre-RFC in the forum first
- Tweak wording around who declares FCP slightly
- Make clear that the shepherds have to announce FCP, and when exactly the period officially starts
- Be more open about when to do the implementation work relative to the RFC, and reflect the current practice of starting implementation work early
- Make more clear that RFCs should not be amended, and that important information should live e.g. in documentation instead.

* RFC template: Use semantic line breaks

Given the short text this may not make a huge difference, but given
that every RFC starts with a copy of that template, the hope is that
it will nudge people to continue writing in that style.

* RFC template: Add "Prior art" section

This is mostly inspired by Rust's RFC template.
Also added a couple of words to the "Alternatives" section,
similarly inspired by Rust's template.

* fixup! Update the RFC process documentation

* fixup! Update the RFC process documentation

Co-authored-by: 7c6f434c <7c6f434c@mail.ru>

* fixup! Update the RFC process documentation

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

---------

Co-authored-by: 7c6f434c <7c6f434c@mail.ru>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

Co-authored-by: Ryan Mulligan <ryan@ryantm.com>
Co-authored-by: Connor Brewster <cbrewster@hey.com>
Co-authored-by: Ben Radford <benradf@users.noreply.github.com>
Co-authored-by: Divam <dfordivam@protonmail.com>

…rimental basis