[develop]: Fix formatting for RTD User's Guide

[gspetro-NOAA]

DESCRIPTION OF CHANGES:

Added the sphinx_rtd_theme extension and pinned docutils==0.16 to resolve RTD formatting issues.

Type of change

• Bug fix (non-breaking change which fixes an issue) --> documentation bug
• This change requires a documentation update

TESTS CONDUCTED:

None required. I built the docs from my fork, and they now render correctly: https://srw-ug.readthedocs.io/en/bugfix-doc-dependencies/Introduction.html

DEPENDENCIES:

N/A

DOCUMENTATION:

N/A

ISSUE:

Issue #532

CHECKLIST

• My code follows the style guidelines in the Contributor's Guide
• I have performed a self-review of my own code using the Code Reviewer's Guide
• I have commented my code, particularly in hard-to-understand areas N/A
• My changes need updates to the documentation. I have made corresponding changes to the documentation
• My changes do not require updates to the documentation (explain). N/A
• My changes generate no new warnings
• New and existing tests pass with my changes N/A
• Any dependent changes have been merged and published N/A

[MichaelLueken]

@gspetro-NOAA Thanks for finding a fix for the RTD formatting issues so quickly! These changes look good to me.