That page is build from phpBB's documentation repository: https://github.com/phpbb/documentation
And then more specifically, the
directory of that repository.
How the 'transformation' is taken place into the actual pages, is done through some Symfony services apparently.
This is not publicly available but in private GitHub repository.
But one has to guess, that it is being build with Sphinx and a custom theme, a theme that corresponds with the rest of the phpBB website.
Some of the
files in the repo can be of more insight, eg: https://github.com/phpbb/documentation/ ... te_docs.sh
Hower, I am just as much in the dark as you are, just listing the possibilities.