Merge branch 'docs/cells-router-latencies' into 'master'

Add latency targets for cells routing layer

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138297



Merged-by: Kamil Trzciński <ayufan@ayufan.eu>
Approved-by: Thong Kuah <tkuah@gitlab.com>
Approved-by: Kamil Trzciński <ayufan@ayufan.eu>
Approved-by: Igor <iwiedler@gitlab.com>
Reviewed-by: Kamil Trzciński <ayufan@ayufan.eu>
Reviewed-by: Steve Xuereb <sxuereb@gitlab.com>
Reviewed-by: Thong Kuah <tkuah@gitlab.com>
Co-authored-by: Steve Xuereb <sxuereb@gitlab.com>