Fix bug where quantum and samples were not showing up in the documentation

[josh146]

Context:

The quantum and samples modules were not showing up in the documentation, since dask was not being installed on readthedocs:

WARNING: autodoc: failed to import module 'quantum' from module 'thewalrus'; the following exception was raised:
No module named 'dask'
WARNING: autodoc: failed to import module 'samples' from module 'thewalrus'; the following exception was raised:
No module named 'dask'

Description of the Change: Adds dask[delayed] to docs/requirements.txt

Benefits: All modules now show in the build documentation.

Possible Drawbacks: Unfortunately, I can't build the documentation locally, I get a Segmentation Fault.

Related GitHub Issues: n/a

[josh146]

(These stylistic changes are just to bring the theme in-line with the latest SF and PL documentation theme)

[codecov]

Codecov Report

Merging #182 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #182   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines         1062      1062           
=========================================
  Hits          1062      1062           

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[nquesada]

Thanks for fixing this @josh146