[docs] Add Rackspace as an Adopter

[cloudnull]

This change adds Rackspace as an adopter of Envoy Gateway, which is curently used within Rackspace OpenStack in production, and as recommended in our open-source platform [0].

[0] https://docs.rackspacecloud.com/infrastructure-envoy-gateway-api/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.30%. Comparing base (1e18ce1) to head (289156d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5712      +/-   ##
==========================================
+ Coverage   65.27%   65.30%   +0.02%     
==========================================
  Files         213      213              
  Lines       34104    34104              
==========================================
+ Hits        22263    22271       +8     
+ Misses      10502    10497       -5     
+ Partials     1339     1336       -3     

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