[3.13] gh-98347: Add links to python-isal in the documentation (GH-98637) (#132894)

gh-98347: Add links to python-isal in the documentation (GH-98637)

Clearly note that this is primarily intended for users for who zlib/gzip is a bottleneck.
(cherry picked from commit b1fc8b69ec4c29026cd8786fc5da0c498c7dcd57)

Co-authored-by: Ruben Vorderman <r.h.p.vorderman@lumc.nl>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>