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

Clearly note that this is primarily intended for users for who zlib/gzip is a bottleneck.

Co-authored-by: Adam Turner <[email protected]>

Author: rhpvorderman

Doc/library/gzip.rst

@@ -258,6 +258,10 @@ Example of how to GZIP compress a binary string::
       The basic data compression module needed to support the :program:`gzip` file
       format.
 
+   In case gzip (de)compression is a bottleneck, the `python-isal`_
+   package speeds up (de)compression with a mostly compatible API.
+
+   .. _python-isal: https://github.com/pycompression/python-isal
 
 .. program:: gzip
 

Doc/library/zlib.rst

@@ -353,3 +353,8 @@ the following constants:
    http://www.zlib.net/manual.html
       The zlib manual explains  the semantics and usage of the library's many
       functions.
+
+   In case gzip (de)compression is a bottleneck, the `python-isal`_
+   package speeds up (de)compression with a mostly compatible API.
+
+   .. _python-isal: https://github.com/pycompression/python-isal