Update Sphinx compile issue after upgrading to Python 3.9 and related packages.

[yhan818]

Summary

RTD https://readthedocs.org/projects/redata-commons/ build success at the workflow (using Python 3.8) under "Actions", but it failed in readthedocs.org website with the latest Pandas 1.4.3.
This is a chain of actions from upgrading from Python 3.7 to Python 3.9. See UAL-RE/redata-commons#23

This is a similar issue to one at redata-commons (UAL-RE/redata-commons#32)
in docs/requirements.txt:
Sphinx 4.0 ==> 5.0.0
sphinx-rtd-theme 0.5.1 ==> 1.0.0
sphinx-autodoc-typehints 1.11.1 ==> 1.18.3

Objectives

Update Sphinx dependencies to the latest version and the same version as other ReDATA repositories, because the latest versions are maintained and previous versions may not be maintained (i.e. issues, security patches).

Proposal

• Update requirements.txt
• Add a new file at the root .readthedocs.yaml
• Update Sphinx dependencies

Testing notes

• Check https://requiam.readthedocs.io/
• Check RTD build status at https://readthedocs.org/projects/requiam/builds/

Additional notes

RTD configuration file https://docs.readthedocs.io/en/stable/config-file/v2.html

Implemented in: TBD