Web docs: update list of C++ idioms that we (don't) use

[rileyjmurray]

Web docs as of RandBLAS 1.1
https://randblas.readthedocs.io/en/1.1.0/FAQ.html#c-idioms-and-features-we-don-t-use
say the only instance methods for structs are constructors and destructors. This is no longer true.

We now have several instance methods for classes that implement SparseMatrix. Those will probably stay around, since we're not worried about cross-language API calls involving SparseMatrix. We also added SketchingDistribution.sample. It's not clear to me whether those instance methods survive into RandBLAS 2.0.