Pull shared dbs out of plan table, explain how they should be used

[bengerman13]

In order to encourage proper use of shared dbs, we want to pull them out of the plan table into their own section with a description of how they're intended to be used

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Security considerations

by better explaining how the shared dbs work, and by moving any mention of them into this description we'll make them less likely to be used for production/sensitive data

Implementation sketch

[links to background notes, sketches, and/or relevant documentation

• remove shared db plans from rds plan table
• add new section with explanation of shared plans