doc: rename Getting Started as User Manual and update the content
- add an installation section (for swh deposit cli)
- do not talk about the metadata-file-less deposit (it's a marginal use case that we do not encourage),
- present the deposit workflow,
- make it a step-by-step reproducible tutorial
Depends on !129 (closed). Related to swh-docs#2751.
Migrated from D4430 (view on Phabricator)