[5pt] Put all included content in _include directories

[NickVolynkin]

Put all included content in _include directories

Reviewer: @NickVolynkin

Sometimes we extract big chunks of documentation to separate files and then include them with .. include::. When we do so, we should put included files in an _include directory next to the parent document.

If we don't, there are several consequenses:

• Included files get built and become separate pages.
• Building has warnings about files not in the toctree.
• Files produce .pot and .po files which go to Crowdin and count as untranslated.

DoD

• Move all included files in _include directories next to parent documents.
• Change paths to include them in the parent documents
• Remove relevant .pot and .po files from the repository. If there are translations in the removed .po files, check if they're also in the parents' .po files. Check that we don't lose translations in the output.
• @NickVolynkin: push to Crowdin to remove files, check results.