* Move some Roda plugins out of server default and into SSR plugin

- Add new documentation on how Roda is configured
- BREAKING CHANGE: the `flash` plugin is no longer loaded by the file-based routes plugin by default, it's now configured with `sessions: true` in the ssr plugin init. Flash required sessions to be set up anyway, which used to be manual but is now provided by this new config.

* Flesh out the Roda reference guide

* A few additional tweaks to Roda docs

* remove unnecessary Rubocop code in docs example