Rationalise the docs

Currently toolbox has three sets of documentation: the readme, the man page, and the [Silverblue docs page|https://docs.fedoraproject.org/en-US/fedora-silverblue/toolbox/]. There's too much duplication here, and we should try and reduce the amount of documentation we're maintaining, in order to improve it.

We should maybe:

- [ ] retire the Silverblue docs page
- [ ] update the man page
- [ ] reuse the man page for the readme
- [ ] copy the readme to https://docs.fedoraproject.org/en-US/docs/