Add custom config to quicktour
This PR updates the quicktour to include a section for building custom configurations that creates a randomly initialized model. Other changes include:

- Added a brief section for `Trainer`.
- Switched back to the code switcher (instead of code blocks) for some code examples which showed essentially the same thing and didn't have drastically different text associated with them. I think this will reduce the amount of scrolling and improve user experience.
- Minor maintenance work to improve conciseness.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Oops, sorry about all the changes! How about we keep the custom config section here and rollback all the other changes, which we can discuss in a separate issue?<|||||>Yes please!