Adding documention for optimizing usage of Cocoapods

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/127364



Merged-by: Fiona Neill <fneill@gitlab.com>
Approved-by: Fiona Neill <fneill@gitlab.com>
Reviewed-by: Fiona Neill <fneill@gitlab.com>
Co-authored-by: Darby Frey <dfrey@gitlab.com>