[JOSS review] Avoid separate doc requirements.txt

[AdamRJensen]

I'm confused as to why there is a separate requirements.txt file for the documentation.

Is there a reason that these dependencies are not listed here:

pvOps/setup.py

Lines 54 to 56 in fe6e657

	DOCS_REQUIRE = [
	'sphinx == 2.2.0'
	]

I see no reason why they should not be included in the DOCS_REQUIRE, which lets you install documentation dependencies the standard way:

pip install pvops[docs]

Also, it's HIGHLY recommended to pin the versions of the documentation dependencies as this often ends up causing trouble down the road!

This is part of a JOSS review: openjournals/joss-reviews#5755

[kbonney]

Addressed by #80. Resolution: removed redundant req files and pinned documentation related dependencies.