Build documentation for PRs in Read The Docs

[mathbunnyru]

This will prevent making changes to documentation which prevent it from building and will allow viewing PR as a rendered page to see if it looks well.

Copied from: #3585 (comment)

Just go to https://readthedocs.org/dashboard/mamba/edit/ (you need to be logged in and have admin access to RTD project, I don't have it), click under Build pull requests for this project, and then Save.

This is how it's going to look like in PRs: mathbunnyru#1