Merge branch 'himkp-reorg-markdown-serializer' into 'master'

Split markdown serializer for marks in separate files

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159630



Merged-by: Enrique Alcántara <ealcantara@gitlab.com>
Approved-by: Enrique Alcántara <ealcantara@gitlab.com>
Co-authored-by: Himanshu Kapoor <hkapoor@gitlab.com>