Docs/vi posterior notebook

[Shamik-07]

Created a new VI notebook and updated the docs.

Note

The q details have been retrieved from here and the vi_method details from here

Important

With my limited understanding of sampling methods and with research, i have included the "when to use VI over MCMC".

Closes #1765 .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.86%. Comparing base (3c60a97) to head (47b0632).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1819   +/-   ##
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  Coverage   87.86%   87.86%           
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  Files         140      140           
  Lines       12726    12726           
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  Hits        11182    11182           
  Misses       1544     1544           

Flag	Coverage Δ
fast	82.72% <ø> (?)

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