implement Bröker's algorithm for constructing supersingular curves

[yyyyx4]

In this patch, we add a function special_supersingular_curve() to construct a supersingular elliptic curve defined over a given $\mathbb F_p$ having a small-degree endomorphism which anticommutes with Frobenius. The function optionally constructs and returns the endomorphism as an explicit isogeny; this should prove useful later when we have better tools for endomorphism rings.

Bröker's algorithm is both the standard approach for finding a supersingular elliptic curve in given characteristic, as well as part of the setup for several algorithms dealing with endomorphism rings: The special endomorphism together with Frobenius forms a generating set of the endomorphism algebra.

Cc: @JohnCremona @defeo @GiacomoPope @remyoudompheng

[JohnCremona]

I amde a couple of comments, not in a formal review (yet).

[yyyyx4]

Thank you!

[vbraun]

pdf docs fail

[sagemath_doc_pdf-none] Overfull \hbox
[sagemath_doc_pdf-none] ! Undefined control sequence.
[sagemath_doc_pdf-none] l.12331 has coefficients in \(\FF
[sagemath_doc_pdf-none]                                  _p\);

[github-actions]

Documentation preview for this PR (built with commit 1e06b22; changes) is ready! 🎉