Update the RFC template

[nrc]

Improve the RFC template by adding more metadata, making the structure more flexible, and tweaking some wording.

Note that this proposal uses the proposed template as a bootstrap example.

Rendered

[apiraino]

@nrc this is an interesting proposal that received some praise :slightly_smiling_face: How far are we from agreeing here? just curious if there are there some important blockers

thanks for an update