Improve the documentation of the RFC process

[lubianat]

This is a first step towards.

• https://github.com/German-BioImaging/ome-zarr-ideas/issues/54
• https://github.com/German-BioImaging/ome-zarr-ideas/issues/55

It may include

• [x] A table documenting the RFC codes

EDIT: other tasks moved to #400 to keep this PR simpler

[github-actions[bot]]

Automated Review URLs

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[clbarnes]

I think this is great! It could go one step further to split the "responsibility" / "onus" into its own column so it's clear whose court the ball is in.