[doc] Expand on soft keywords in the docs for keyword.py (GH-29139) (GH-29195)

Add link at the top and fix the existing links to point to the "[soft keywords](https://docs.python.org/3.10/reference/lexical_analysis.htmlGH-soft-keywords)" section created in the Python 3.10 docs.

(cherry picked from commit 9e05da62241877a871c7b22ef723d7028ce03c90)

Co-authored-by: William Andrea <william.j.andrea@gmail.com>