Add webhook branch filtering docs
What does this MR do?
Document that push event web hooks can be filtered server side.
Related issues
https://gitlab.com/gitlab-org/gitlab-ce/issues/20338
Author's checklist
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Apply the correct labels and milestone -
Crosslink the document from the higher-level index -
Crosslink the document from other subject-related docs -
Correctly apply the product badges and tiers -
Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-checkjob fails
Review checklist
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Your team's review (required) -
PM's review (recommended, but not a blocker) -
Technical writer's review (required) -
Merge the EE-MR first, CE-MR afterwards