[getting-started]: add db baselining chapter

[ruheni]

Describe this PR

• Added a chapter for baselining a database using prisma migrate diff
• Added Tip for migrating from other ORMs (both relational dbs and MongoDB)

Changes

What issue does this fix?

Any other relevant information

[vercel[bot]]

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Name	Status	Preview	Updated
docs	✅ Ready (Inspect)	Visit Preview	Nov 15, 2022 at 6:27PM (UTC)

[github-actions[bot]]

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

Changed Vercel links

• /docs/getting-started/setup-prisma/add-to-existing-project/relational-databases/introspection (currently)
• /docs/getting-started/setup-prisma/add-to-existing-project/relational-databases/baseline-your-database (currently)
• /docs/getting-started/setup-prisma/add-to-existing-project/relational-databases/install-prisma-client (currently)
• /docs/getting-started/setup-prisma/add-to-existing-project/relational-databases/querying-the-database (currently)
• /docs/getting-started/setup-prisma/add-to-existing-project/relational-databases/evolve-your-schema (currently)
• /docs/getting-started/setup-prisma/add-to-existing-project/relational-databases (currently)
• /docs/getting-started/setup-prisma/add-to-existing-project/mongodb (currently)

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

[tanberry]

Thanks @ruheni for writing this up! Can we have a discussion about my Slack questions concerning possible duplication of content, and other Qs?