Attempt to fix readthedocs build

[daico007]

PR Summary:

For some reason, the docs build on PR has been failing (for a while). This PR will attempt to find and fix that issue.

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -1.87 ⚠️

Comparison is base (d1c1f33) 86.89% compared to head (fe7965e) 85.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1105      +/-   ##
==========================================
- Coverage   86.89%   85.02%   -1.87%     
==========================================
  Files          61       61              
  Lines        6318     6319       +1     
==========================================
- Hits         5490     5373     -117     
- Misses        828      946     +118     

Impacted Files	Coverage Δ
mbuild/recipes/__init__.py	100.00% <100.00%> (ø)
mbuild/utils/io.py	83.62% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[daico007]

Ok, I think the docs (somehow) is building correctly now. There were a combinatotiral issue with the fail_on_warning clause in .readthedocs.yml compounded with some deprecated warnings, missing dependencies, etc. But I hand checked the built docs and all pages seems to be built alright (especially those writers which are built from their docstrings). So, I think this PR is ready for review, if you want to have a look at it @chrisiacovella, if not I can merge it either tomorrow or wednesday.

Also, we have an DeprecatedWarnings regading the pkg_resrouces, which is currently used in mbuild/utils/io.py and mbuild/recipes/__init__.py that we might also want to take care of. I will try to poke a round and see what's the updated syntax should be and push it in later.