Document `only/except: external_pull_requests` CI yaml syntax

References

Merged MR that introduced documentation requiring review:

Related issue(s): https://gitlab.com/gitlab-org/gitlab-ce/issues/65139

Further Details

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15082 we introduce a new syntax only/except: external_pull_requests which enables pipelines to run in the context of an external pull request (e.g. GitHub pull request).

In this issue we need to document:

• preconditions for its usage
• how it works
• additional predefined variables
• limitations
• describe some use cases
• difference with only/except: branches ? describe some gotchas as per https://gitlab.com/gitlab-org/gitlab-ee/issues/24089