Update docs to use Prisma file

[infomiho]

This is a quick pass through all the code blocks that used the entity declaration. They are now converted to use the Prisma file.

This should be the main docs branch where we merge other work like:

• [x] Updating the data model pages https://github.com/wasp-lang/wasp/pull/2130
• [x] Write a new Prisma configuration page https://github.com/wasp-lang/wasp/pull/2131
• [x] Updating the tutorial pages https://github.com/wasp-lang/wasp/pull/2134
• [ ] Updating the "Accessing User Data" page
• [ ] Writing the migration page https://github.com/wasp-lang/wasp/issues/2127
• [ ] Updating all other pages