DOC: add a bare bones readthedocs yaml file

[wkliao]

Add init files for readthedocs integration.

• .readthedocs.yaml is required for GitHub CI to trigger readthedocs to build
  documents.

[wkliao]

Hi, @carns

This PR adds a few init files for readthedocs.
On readthedoc dashboard, it showed the first build successful.
If this PR looks OK with you, please approve it, so PR #1033 can be triggered.

This bare-bones PR created a minimal document on readthedocs.io,
See it in https://darshan--1034.org.readthedocs.build/en/1034/

[carns]

thanks!