Get Started guide for MongoDB, clarify connection string

[tanberry]

The content about connection URLs in the Get Started guides (both from scratch and existing) for MongoDB is a confusing mishmash of Atlas-centric commands and examples with non-Atlas commands.

Let's figure out a way to present the proper instructions and examples for both, but keep them separate on that page. Maybe a tabbed section? Or a tech switcher?

See this Slack thread for an issue about needing to append ?authSource=admin to their connection string if NOT using Atlas.

See this Slack thread for some info about the error that one gets with Atlas, if you simply copy/paste the connection string from the MongoDB UI (without adding in the database name).

See this Prisma issue where a user ran into the problem of not knowing to add the database name in (if using Atlas).

[keerlu]

Linked to #3306