Update the "Composing applications with diesel" guide to diesel 2.0

[weiznich]

As preparation of the upcoming diesel 2.0 release we need to update the guides on our web page to reflect the corresponding changes.
For the "Composing Applications with Diesel" guide this involves the following points:

1. Update any code snippet to diesel 2.0. Unfortunately most of them directly link to the crates.io code, so a "real" update is required here
2. Remove the links to the crates.io source code, as they use diesel 1.4.x currently. Hopefully we can add them back as soon as they've updated their code.
3. Update any documentation link to point to the corresponding 2.0.x documentation page
4. Open a PR with all this changes targeting the diesel_2.0_update branch

Please use this issue as discussion platform to ask any question that is unclear about updating this guide.

[weiznich]

Fixed with the 2.0 release