Merge branch 'rd-rest-api-move-third-party-client-docs' into 'master'

REST API - move third-party client docs to a separate page

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167119



Merged-by: Suzanne Selhorn <sselhorn@gitlab.com>
Co-authored-by: Russell Dickenson <rdickenson@gitlab.com>