Make it explicit that the dependency list
is generated from Gemnasium.

Show how one can switch to Python 3.5.

Also slight edit and spelling correction

This updates our build images to use node@12. We are also adding a
sentence to the from-source installation that while we require a minimum
version of node@8, we also recommend node@12. Last but not least, we
change the version of node in the .nvmrc so that developers using nvm
can automatically use node@12 with the project

See http://asciidoctor.org/docs/user-manual/#include-directive

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13763

Also update the Review Apps node pools description
Signed-off-by: NRémy Coutable <remy@rymai.me>

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62505