docs/Project.toml: set sources path for Nemo

[fingolfin]

This makes it a tad more convenient to build the docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.11%. Comparing base (c43dcc7) to head (fb1b351).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2147      +/-   ##
==========================================
- Coverage   88.15%   88.11%   -0.05%     
==========================================
  Files          99       99              
  Lines       37053    37053              
==========================================
- Hits        32664    32649      -15     
- Misses       4389     4404      +15     

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[lgoettgens]

Can we please first finish the discussion at Nemocas/AbstractAlgebra.jl#2173?

[fingolfin]

I am not sure why this should wait for Nemocas/AbstractAlgebra.jl#2173 -- that one is a bigger change, this one is just to make the existing practice of building the Nemo manual a bit easier. Anyone wanting to build the manual currently has to remember to first dev PATH/to/NEMO with this patch that steps is removed

[fingolfin]

The real analogue is Nemocas/AbstractAlgebra.jl#2175

[lgoettgens]

Just as a note: The [sources] functionality was just introduced in julia 1.11 (see https://github.com/JuliaLang/Pkg.jl/blob/master/CHANGELOG.md#pkg-v111-release-notes)