[docs] Remove end-user content from security README

[tiffany76]

Description

The end-user security documentation was moved to the OTel website in open-telemetry/opentelemetry.io#5209 and open-telemetry/opentelemetry.io#5729. This PR removes the end-user content from security-best-practices.md, leaving only the component developer documentation.

Link to tracking issue

Fixes open-telemetry/opentelemetry.io#3479

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.62%. Comparing base (ffcef93) to head (7cfcc2a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12002   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         447      447           
  Lines       23739    23739           
=======================================
  Hits        21751    21751           
  Misses       1613     1613           
  Partials      375      375           

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