doc: add use cases and benchmark description
Check-in the final version of the use cases / benchmark document, originally written in swh/meta$458 for #1885 (closed).
In this version I've:
- integrated the latest changes from @grouss (with minor editing)
- removed the obsolete links to API proposals that have been implemented since
- converted the markup to rst, for consistency with our Python documentation
Migrated from D1738 (view on Phabricator)