Update docs

[thomaspinder]

Improve the documentation standards. It also makes the documentation's build process more robust.

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

The existing documentation is adequate but far from perfect. This PR seeks to include more worded descriptions, mathematical annotations and code examples to the existing docstrings.

Issue Number: #136

[codecov]

Codecov Report

Merging #137 (ddfbfab) into v0.5_update (7a51a9e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           v0.5_update     #137   +/-   ##
============================================
  Coverage        98.27%   98.27%           
============================================
  Files               15       15           
  Lines             1331     1332    +1     
============================================
+ Hits              1308     1309    +1     
  Misses              23       23           

Flag	Coverage Δ
unittests	98.27% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gpjax/gps.py	99.35% <100.00%> (+<0.01%)	⬆️
gpjax/kernels.py	95.08% <100.00%> (ø)

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