Add documentation for command-line tools

Test Plan

make -C docs html adds a new file

---

Migrated from D3028 (view on Phabricator)

docs/cli.rst

+Command-line interface
+======================
+
+Shared command-line interface
+-----------------------------
+
+.. click:: swh.core.cli:swh
+  :prog: swh
+  :show-nested:
+
+Database initialization utilities
+---------------------------------
+
+.. click:: swh.core.cli.db:init
+  :prog: swh db init
+  :show-nested:
+
+.. click:: swh.core.cli.db:db_init
+  :prog: swh db-init
+  :show-nested: