Add howto doc for path-regex annotations

[jjngx]

Proposed changes

This PR adds a document guiding users how to apply nginx.org/path-regex annotations to simple and mergeable ingresses.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[codecov]

Codecov Report

Merging #4266 (ea826db) into main (25b9e05) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4266      +/-   ##
==========================================
- Coverage   51.97%   51.95%   -0.03%     
==========================================
  Files          59       59              
  Lines       16762    16762              
==========================================
- Hits         8712     8708       -4     
- Misses       7753     7755       +2     
- Partials      297      299       +2     

see 1 file with indirect coverage changes

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[ADubhlaoich]

Approved, but I made a lot of suggestions about nouns, prepositions and articles.

There's also a lot of missing punctuation for the end of instruction steps.

[brianehlert]

LGTM