Re-export alga docs to improve discoverability of trait implementations

[jswrenn]

Implements the suggestion I made in #511 (comment).

This provides a one-stop place to find out which traits from alga are implemented for types from nalgebra.

For example, here's how rustdoc renders alga_documentation::linear::AffineTransformation:

Once doc_cfg is stabilized, the deprecated notice should be superceded (or augmented) by #[cfg(rustdoc)].

[Andlon]

Since nalgebra these days is based primarily upon simba, my understanding is that this PR is no longer relevant. I'll therefore close it.