Describe the external contributions process in more details

Currently there's no good documentation on the community contributions process.
There's scattered bits in https://mariadb.com/docs/general-resources/community/contributing-participating/contributing-code. And then https://github.com/MariaDB/server/blob/main/CONTRIBUTING.md suggests to "talk to the MariaDB developers". This is not optimal.

We need a proper external contribution howto, documenting each step of the way: from how to go about it, what is needed, what steps are there and what to expect at each step.

But putting all that information into CONTRIBUTING.md itself will bloat it significantly and dilute its message.

IMHO this is best captured IMHO in a standalone .md file (e.g. COMMUNITY_CONTRIBUTIONS.md) that's referenced from contributing.md and from the plugins/ page.

This should describe it all: state diagram, step by step howto etc.