@akorotkov I'm interested also. I see the forked and patched version here, https://github.com/orioledb/postgres, but I don't see any documentation on what changes you made.
There is not exactly a documentation. But there you can see that commits after "Stamp 14.2." are extendability patches. Commit messages briefly describe the changes.
https://github.com/orioledb/postgres/commits/patches14
Patches will be polished and published on pgsql-hackers.