PostgreSQL extensions documentation

[keerlu]

Describe this PR

Draft of the Postgres extensions preview feature (support for representing extensions in your schema)

Changes

What issue does this fix?

Fixes #3836

Any other relevant information

[vercel[bot]]

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Name	Status	Preview	Updated
docs	✅ Ready (Inspect)	Visit Preview	Oct 18, 2022 at 9:42AM (UTC)

[github-actions[bot]]

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

Changed Vercel links

• /docs/concepts/components/prisma-schema/postgresql-extensions (currently)
• /docs/reference/api-reference/prisma-schema-reference (currently)

(Note that links will only be valid after Vercel preview deploy succeeded)