Overhaul, update, unify documentation for creating, submitting, maintaining, contributing to, ... etc. GAP packages

[fingolfin]

We have lots of information on this but it is spread over too many places.

• chapter 76 of the reference manual
• several pages on the website (see also issue #5382)
  • https://www.gap-system.org/packages/authors/
  • https://www.gap-system.org/packages/authors/variablenames/
  • https://www.gap-system.org/packages/authors/submit/
• the example package (resp. its PackageInfo.g file
• some are also just not documented anywhere, see e.g. #4821 or #4987
• ... more I can't think of right now but will at later.

Also, this documentation is not very coherent. Some of it is a reference, some a HOW-TO, some explanations, but all jumbled together (see also https://docs.divio.com/documentation-system/ for hints about organizing documentation)

It would be great to improve the state of this. I don't think that means merging everything into chapter 76 of the manual. Actually I'd rather split that into multiple chapters or even move (most of) it to a separate document, with a better logical structure...