Update the getting started 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 "Getting Started" guide this involves the following points:

1. Update any code snippet to the corresponding variant from the 2.0.x branch
2. Update any link pointing to the corresponding example in our repository to point to the 2.0.x branch
3. Update any documentation link to point to the corresponding 2.0.x documentation page
4. Update the note about the minimal supported rust version to state that at least rust 1.54 is required
5. Update the note about embed_migrations! to the new syntax used there
6. 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 by #158