Add guidelines for new algorithms (docs and skeletons)

[albcab]

Rough draft of the developer docs, closes #235.

I've put some design principles on writing. The idea is to not only write the developer documentation but at the same time evaluate our current API, specify the main objectives of the library and where we want it to be in the future, so that we can think about [if, when and how] we want to restructure.

Opinions, comments, critiques, things I've not considered, etc. are appreciated!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.86%. Comparing base (bc89395) to head (ff87bac).

Additional details and impacted files

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  Coverage   98.86%   98.86%           
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  Files          59       59           
  Lines        2741     2741           
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  Hits         2710     2710           
  Misses         31       31           

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[gil2rok]

Found this guide to be very helpful! I really like the skeleton template code for new samplers!

[gil2rok]

I saw a small, possible mistake in the VIAlgorithm class documentation that may be relevant here. I described it in #700 .

I also still really like this helpful doc.

[johannahaffner]

Will this be merged and is it still up to date?

[albcab]

Will this be merged and is it still up to date?

It should be up to date, but if you find anything outdated, just push a commit (or let me know)!