Make building the package docs more convenient

[fingolfin]

The sources entry in docs/Project.toml helps ensure the correct version of the package is used.

There is also now a way to override the doctest test setting, so that one can turn them off for a quick test, or enable the fix mode for updating examples.

This may need some discussion before it gets merged and rolled out to perhaps some more of our repos; I'll add some inline comments to relevant code sections

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 87.93%. Comparing base (4b127f8) to head (f51792d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2173   +/-   ##
=======================================
  Coverage   87.93%   87.93%           
=======================================
  Files         127      127           
  Lines       31776    31776           
=======================================
  Hits        27941    27941           
  Misses       3835     3835           

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[thofma]

What about the Hecke.build_doc()/Oscar.build_doc() way? Just wondering what you mean when you say "rolled out to perhaps some more of our repos". Do we want two slightly different ways for the same thing?

[fingolfin]

I don't like the Hecke.build_doc()/Oscar.build_doc() way -- it causes us all kinds of pain, and I think we can get the same or better with less hacks, based on what this PR does plus more.

[fingolfin]

(In particular I have an extended version of this using Revise and for repeated rebuilds of the docs. I'll try to get it cleaned up and into another PR, but I think it might actually be better to do one for Hecke or Oscar first. We'll see. Might also be a good idea if @thofma @lgoettgens and me sat on a Zoom (?) call for a bit to talk about it. But we are all busy and this is hardly a priority right now...

[fieker]

What is is "new" idea how to build the documentation? As in how and what command?

[thofma]

Without Revise support I have a hard time imagening how doing julia docs/make.jl (which is what this PR here adivses?) is better than *.build_doc(). Can you just add the Revise support here in this PR?