Fix explicit m:n SQL

[janpio]

The SQL was missing the actual primary key of CategoriesOnPosts and was using a unique instead. Some of the fields on the models were also missing.

Additionally the SQL was not like what current Prisma Migrate would create. Note that this is more verbose and might not be what we actually want to show here. But at least this is correct now.

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This PR changes the following pages (Vercel Preview Deploy links):

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• /docs/concepts/components/prisma-schema/relations/many-to-many-relations (currently)

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[tanberry]

Thanks @janpio . Do we need to simultaneously clean up the diagram you mentioned in Slack? Or can they be separate improvements?

[janpio]

This is completely independent from any messages in Slack.

[andrew-walford-prisma]

@keerlu This is a PR I'd otherwise merge, but it has a merge conflict with one of your docs, so I thought I'd leave this for you to decide on!