Add documentation pull request preview via Read the Docs

[stevepiercy]

Do we have a docs pull request preview, such as that provided by Read the Docs? It would be good to verify that RFC links, API docs, and other markup renders correctly, without pulling down the branch and building the docs locally.

If not, we could copy what I did for plone-sphinx-theme. See also https://github.com/plone/plone-sphinx-theme/blob/main/.readthedocs.yaml#L13-L25 and https://github.com/plone/plone-sphinx-theme/blob/main/Makefile#L215-L217 for complete example.

Originally posted by @stevepiercy in #667 (comment)

[niccokunzmann]

This sounds good to me!

[stevepiercy]

Ref: Pull request previews and How to configure pull request builds.

[stevepiercy]

@niccokunzmann would you please add me as a Maintainer for the RTD project? I am stevepiercy on RTD.

https://readthedocs.org/projects/icalendar/

Else please follow the instructions for How to configure pull request builds. Thank you!

[niccokunzmann]

@stevepiercy: Maintainer for the RTD project?

Done!
And I enables the checkbox for the PR build, too. Thanks for the link!