docs: a nicer readme

[borgoat]

Summary

I made the readme more approachable, and started splitting more "internal docs"
like that about releasing.

Impact

• App
• Supabase functions
• Supabase migration
• CI / GitHub
• pubspec.yaml

Open points

I'd like to improve the dev docs next, adding some stuff about the internal design, the schema, the codebase structure... for now, I just split it to keep the readme lean

[inpt333]

Nice! Just found one small thing, for some reason in the preview with wide screen the mobile image is misaligned. I thought the idea was to have it aligned left, but it's not like that either:

In any case, it's possible that it's not reproducible in GitHub since the allocated space for the README is not wide enough.

[borgoat]

The plan was indeed to have it left aligned with some text to the right (similar to www.grup.rsvp) but I couldn't get it to work. I believe because of the picture tag... no idea why it's not always centred either though..,