[Documentation] Add documentation for instance level external destinations

Add documentation for changes related to API and UI introduced for adding, listing, updating and deleting instance level external audit event destinations by issues #398107 (closed) and #335175 (closed).

added to epic &6125 (closed)

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assigned to @hraghuvanshi

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added Category:Audit Events Deliverable Next Up automation:ml automation:ml wrong backend customer devopsgovern documentation groupcompliance missed-deliverable missed:15.10 missed:15.11 missed:16.0 priority2 sectionsec typefeature workflowcomplete labels

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mentioned in merge request !122898 (merged)

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@hraghuvanshi This issue looks like it may slip this current milestone. Can you leave a or to signify if you are on track to deliver this issue? Please also consider updating the issue's Health Status or Milestone to reflect its current state, and communicate with your Product Manager as appropriate.

Bot policy.

added workflowin review label and removed workflowin dev label

mentioned in issue #413683 (closed)

mentioned in issue #413681 (closed)

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Closing this as MR merged and docs are being further updated here Split audit event streaming documentation into ... (!123336 - merged)

closed

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mentioned in issue gitlab-org/govern/compliance/general#118

changed epic to &11062 (closed)