Document the structure of the project

[bchapuis]

The documentation contains TODOs that have not been completed.
https://baremaps.apache.org/documentation/developer-manual/project-structure/

I wouldn't go into much details as the structure and the modules tend to evolve, but a high level description may be beneficial. For instance, we could give a broad description for:

• baremaps-core,
• baremaps-cli,
• why we have other submodules for codecs (baremaps-openstreetmap, baremaps-pmtiles, etc.).

As said, I wouldn't go into the details as the structure evolves, the intend should be to give pointers to new commers.