bpo-32614: Modify re examples to use a raw string to prevent warning (GH-5265) (#5499)

Modify RE examples in documentation to use raw strings to prevent DeprecationWarning.
Add text to REGEX HOWTO to highlight the deprecation.  Approved by Serhiy Storchaka.
(cherry picked from commit 66771422d0541289d0b1287bc3c28e8b5609f6b4)

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
4 files changed