explain c++ and python repo split in documentation

[TimmRuppert]

Reference to a related issue in the repository

#888

Add a description

I’ve drafted an initial version of the documentation explaining the new repository structure, including the removal of bindings from the core repo. I’ve long been frustrated with the quality of the OSI installation and linking documentation, so I took the liberty of reworking that section. While I’m still not completely satisfied (I struggle to provide a user-friendly version), I’d like to get feedback first to see if people agree with this direction before proceeding further.

Take this checklist as orientation for yourself, if this PR is ready for the Change Control Board:

• My suggestion follows the style and contributors guidelines.
• I have taken care about the message documentation and the fields and enums documentation.
• I have done the DCO signoff.
• My changes generate no errors when passing CI tests.
• I have successfully implemented and tested my fix/feature locally.
• Appropriate reviewer(s) are assigned.