Add guide for TLS Route in site

[sarthyparty]

Proposed changes

Problem: No user-facing documentation for configuring TLS Passthrough

Solution: Add a how-to guide for TLS Passthrough and update Gateway API compatibility doc

Closes #2285

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

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Add how-to user guide for configuring TLS Passthrough load-balancing

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.81%. Comparing base (16a9522) to head (3ac39b2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2363      +/-   ##
==========================================
- Coverage   88.84%   88.81%   -0.03%     
==========================================
  Files         100      100              
  Lines        7527     7527              
  Branches       50       50              
==========================================
- Hits         6687     6685       -2     
- Misses        784      785       +1     
- Partials       56       57       +1     

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[sjberman]

We should include something more here to indicate that it's actually working if possible, other than just a response.

[ADubhlaoich]

LGTM, generally. See Saylor's feedback about the DOCS code.

My edit suggestions are largely to remove possessive language, which we avoid for tone.

[nginx-bot]

Hi @sarthyparty!

Thanks for opening this pull request!
Be sure to check out our Contributing Guidelines while you wait for someone on the team to review this.