crates: Add a developer documentation at module level
Mainly move documentation content from docs/user to crates module (See swh-docs!298 (closed) for details)
Related T4104
Migrated from D8206 (view on Phabricator)
Mainly move documentation content from docs/user to crates module (See swh-docs!298 (closed) for details)
Related T4104
Migrated from D8206 (view on Phabricator)