Docs improvements

[scarrazza]

Here we include some extra subsections to simplify the navigation from the user point of view.

[codecov]

Codecov Report

Merging #399 (9531b36) into simtf (acf7c19) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             simtf      #399   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           80        80           
  Lines        12731     12731           
=========================================
  Hits         12731     12731           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/qibo/abstractions/gates.py	100.00% <ø> (ø)
src/qibo/hamiltonians.py	100.00% <100.00%> (ø)

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[scarrazza]

@stavros11 I would like to merge this before completing the docs in #398, could you please let me know if you agree?

[stavros11]

Thanks for writing this, I agree with having a seperate section for each model / gate / Hamiltonian, as this makes it easier for the user to navigate the docs through the side contents. Some small comments below.

Also remember to change the base branch to simtf before merging this.