Broken link checks removed from Content Release

[timcosgrove]

Requirements

Broken link checks should be removed from Content Release, so that content types being removed from Content Build and into Next Build does not trigger broken link alerts.

Acceptance criteria

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1. Github Action that runs broken link checking script and notifies slack with results #16824
   0 of 2
   Accelerated Publishing +1
   
2. Github action broken link checker is signed off on as providing coverage.
3. Flag is implemented on Content Build which disables broken link checks (false by default, i.e. broken link checks proceed unless flagged)
4. Update any documentation around how broken link checks operate

Background & implementation details

External Broken Link checking must be implemented prior to this removal, and determined to provide satisfactory coverage for broken links.

The work here should allow broken link checking to be disabled via a flag, rather than removing it entirely.