Simplify numba mocking functions and improve docs

[matteobachetti]

This simplifies the creation of mock numba decorators, going to the barebones of the decorator behavior and avoiding duplication. It still works as expected, and I suspect (I'm not 100% sure) that it will help estimating coverage in numba-compiled functions during our test runs.

I also added a section on dependencies in the docs. I see that people do not often realize how useful it is to install numba!

[codecov]

Codecov Report

Merging #731 (55ee6e0) into main (1c85c6b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #731   +/-   ##
=======================================
  Coverage   97.15%   97.16%           
=======================================
  Files          42       42           
  Lines        7852     7843    -9     
=======================================
- Hits         7629     7621    -8     
+ Misses        223      222    -1     

Impacted Files	Coverage Δ
stingray/utils.py	98.46% <100.00%> (+0.16%)	⬆️

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[dhuppenkothen]

LGTM overall. I like the extra information in the docs. two questions about package dependencies, but otherwise I think this can get merged. :)