fix: rename MR-VPP ATBD folder/file to underscore separator

Renamed DOCS/MRVPP/MRVPP-ATBD_v2/ -> DOCS/MRVPP/MRVPP_ATBD_v2/ (folder,
.qmd, and -media/ folder) so the ATBD matches the underscore-separated
naming already used by the sibling MRVPP_PUM_v1 document. Updated the
14 internal media-path references inside the .qmd accordingly.

Not yet published to main/gh-pages, so no public URL is broken by this
rename.

Author: MatMatt