Plan how community modules can be linked to the Hakunapi core repository

[teezip]

Different alternatives should be analyzed:

1. this repository contains only a list of external community modules (that support a specific Hakunapi release, that should be also mentioned on the list), the Hakunapi repository does not have any dependencies to such modules
2. the repository contains community/modules etc folder to which community modules would be added via contributions (pull requests), such modules would be testable when testing and packaging core Hakunapi modules, but embedding them to Hakunapi server deployments would be optional
3. some other?

Also some of module currently in https://github.com/nlsfi/hakunapi/tree/main/src could be also moved to community module section, like "hakunapi-esbulk", "hakunapi-smile", "hakunapi-telemetry-opentelemetry", that are not widely used and not under active development.

Currently there is CONTRIBUTING.md document that guides how to give feedback, report bugs, reporting security issues, suggest enhancements or contributing a pull request.

This document should be edited to contain also guidance about community modules.