docs: Add new conceptual pages for intro concepts

[melsal13]

What type of PR is this?
Created new concept pages for:

• BTP
• CTP
• SP
• Proxy
• Gateway API
• API Gateways
• Gateway API Extensions

What this PR does / why we need it:
Introduces foundational conceptual docs to help onboard new users and improve documentation clarity.

Which issue(s) this PR fixes:
Fixes #2070
Fixes #5949
Fixes #2072
Fixes #5948
Fixes #5738
Fixes #5753
Fixes #5754

Release Notes: No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.83%. Comparing base (0752df1) to head (2049c3d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5981   +/-   ##
=======================================
  Coverage   65.82%   65.83%           
=======================================
  Files         217      217           
  Lines       36063    36063           
=======================================
+ Hits        23738    23741    +3     
+ Misses      10845    10843    -2     
+ Partials     1480     1479    -1     

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