docs: add gateway namespace mode documentation

[cnvergence]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #5971

Release Notes: Yes/No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.40%. Comparing base (3e1bce3) to head (1fc9c9f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6040      +/-   ##
==========================================
- Coverage   70.43%   70.40%   -0.04%     
==========================================
  Files         217      217              
  Lines       36129    36129              
==========================================
- Hits        25447    25436      -11     
- Misses       9166     9174       +8     
- Partials     1516     1519       +3     

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[arkodg]

LGTM thanks !