Convert MongoDB from set to book

[haszi]

This is the doc-base part of converting the MongoDB extension from a <set> to a <book> as its top level element.

Please note that for this PR to work documentation repositories will need to make the same changes as in the accompanying doc-en PR.

[cmb69]

I've send an email to the documentation ML; maybe we can make the work of translators a bit easier.

[haszi]

I've opened a PR in every translation's repo. If all this is acceptable, please merge this PR followed by https://github.com/php/doc-en/pull/3627 first.