move small sections into YAML front matter

This makes it more immediately visible to users reading the proposal by email or just browsing the website. Things like "affected users", "approval" and "deadline" are particularly things that rarely require a whole paragraph to describe.

This also makes the contents greppable, which is particularly relevant for the Status field, which could now be used for automatic generation.

That front matter is not currently very well displayed by GitLab wikis, but it's relatively readable.

I have tried to move the "Summary" in there as well, but the formatting gets ugly quickly, as we need to use line continuation hacks. Besides, it looks better in the wiki that way.

This only changes one page for now, but the idea would be to change the template and all existing RFCs as well.