swh-storage: `make distclean` in docs/ deletes committed files

docs/images/swh-archive-copies.pdf docs/images/swh-archive-copies.svg

get deleted when doing make distclean in swh-storage/docs.

Migrated from T1269 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information