Add tests for NOT_SUPPORTED and extend docs (supported methods per engine)

[lukaszstolarczuk]

We should add tests and probably extend documentation with information which methods are available in which engines.

As for documentation:

• we could make some static table with information which methods are delivered with which engine (or per functionality, e.g. for sorted engines, ...),
• we could generate docs automatically and make auto PR with changes,
• ...

As for tests:

• we could make a binary for each function (group of functions) and test it separately per engine,
  • and here we could make the list hard-coded per engine e.g. in CMake,
  • or we could run it generically and based on returned statuses generate the list of supported methods (and generate the docs BTW),
• ...

// Partially done in https://github.com/pmem/pmemkv/pull/904

[karczex]

Probably we should add page on pmem.io