docs: remove scg related docs

[maelle]

Fix #1109

One of the commits is from #1113

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[maelle]

 @szhorvat where in the reference would stochastic_matrix() fit? A new subsection?

[szhorvat]

The stochastic matrix is related to other graph matrices such as the adjacency matrix and the Laplacian matrix. It makes sense to include these three together in the same section.

[maelle]

"Laplacian matrix" is under "Structural properties" whereas as_adjacency_matrix() is under "Conversion". Should I put stochastic_matrix() under "Structural properties"? Thank you!

[szhorvat]

Let's put a "Matrices" subcategory under Structural Properties and put both the Laplacian and stochastic matrices there. If it is possible to include functions in more than one category, then let's also include as_adjacency_matrix() there (in addition to Conversion).

[aviator-app]

This pull request can't be queued because it's currently a draft.

[maelle]

I'll rebase after #1113