Fix 'readthedocs'

[GeorgeGayno-NOAA]

DESCRIPTION OF CHANGES:

• Add a new .readthedocs.yaml file.
• Remove references to sphinxconfig.bibtex, which is not currently used by our documentation.

TESTS CONDUCTED:

All done using the branch at ed637cc.

• Compile branch on all Tier 1 machines using Intel (Orion, Jet, Hera and WCOSS2).
• Compile branch on Hera using GNU.
• Compile branch in 'Debug' mode on WCOSS2.
• Run unit tests locally on any Tier 1 machine. Done on Hera.

Consistency tests were not run as no source code was changed.

DEPENDENCIES:

None.

DOCUMENTATION:

ReadTheDocs was successfully built: https://georgegayno-noaaufs-utils.readthedocs.io/en/bugfix-bnu_soil/

Doxygen was built with no issues.

ISSUE:

Fixes #860

CONTRIBUTORS:

With thanks to @gspetro-NOAA

[GeorgeGayno-NOAA]

@gspetro-NOAA - your review is welcome.