docs/repod/installation.rst:
Mention `repod-file` instead of `db2json`/ `json2db` as the latter are
removed.

docs/repod/tooling.rst:
Replace the documentation for `db2json`/ `json2db` with new
documentation on the various subcommands of `repod-file`.

docs/repod/tooling.rst:
Add documentation on the existing tooling `db2json` and `json2db`.