Docs build improvements

[jklaise]

This PR does 2 things:

• Split out the docs build step in CI as it doesn't have to be done for every Python version and also separates the requirements from the test environment more cleanly
• Add a config file for readthedocs so the builds are configured directly in the repo instead of via their console

[codecov]

Codecov Report

Merging #353 (14f9199) into master (656a5eb) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
- Coverage   86.20%   86.08%   -0.12%     
==========================================
  Files          56       56              
  Lines        7219     7222       +3     
==========================================
- Hits         6223     6217       -6     
- Misses        996     1005       +9     

Impacted Files	Coverage Δ
alibi/explainers/ale.py	59.69% <100.00%> (+0.62%)	⬆️
alibi/explainers/anchor_base.py	89.24% <0.00%> (-2.85%)	⬇️

[jklaise]

Added a version guard around typing_extensions module (only necessary for Python < 3.8). This should've been done as part of #349 but it is actually pulled in by some 3rd party dependency on Python 3.8 hence we didn't see an error.