Restore the umask note for building docs

[oraNod]

Bring back the details about umask 0022 for building docs. https://github.com/ansible/ansible-documentation/blob/9f02e505d94bf402cfcc268efb3ace42dc2de4b7/docs/docsite/rst/community/documentation_contributions.rst#setting-up-your-environment-to-build-documentation-locally

[gotmax23]

What's the rationale here?

[samccann]

This was fixed by #1400

afaik this is required to build the docs locally. It was in the docs for the longest time but got removed and a user /contributor got stuck by this so we added it back.