[REF229] integrate doc build with readthedocs

• Original Gitlab task: https://code.ornl.gov/sns-hfir-scse/reflectometry/reflectometry/-/issues/229

This MR modifies the doc building settings slightly so that the documentation service provider (readthedocs) can build the doc page with latest version.

• The doc build history can be found at: https://readthedocs.org/projects/web-reflectivity/builds/16154362/
• The built doc can be viewed at: https://web-reflectivity.readthedocs.io/en/ref229_doc_pipeline/

additional settings change is necessary to enable the integration between this repo and readthedoc.