fix(databases): Improve documentation for adapters and allow dynamic Knex adapter options

[daffl]

This pull request reviews and improves the MongoDB and Knex adapter documentation.

For the MongoDB adapter:

• Fixes documentation for transactions
• Adds documentation for adding indexes
• Updates the data handling section to focus on ObjectIds with examples that can be dropped directly into a generated application
• Changes date handling suggestion to use timestamps instead of date strings and objects
• Removes the "common mistakes" section (this will eventually be addressed in WIP: Docs "Troubleshooting" Guide #2820)

For the SQL Knex adapter:

• Updates all the documentation to be in line with the current implementation
• Adds documentation for missing methods
• Adds documentation for associations and related entries
• Fixes the setting of dynamic options

Addresses the following issues:

• Closes How to provide unique index property in feathers dove schema just like feathers dove mongoose models  #2791
• Closes [feathers-dove] - How to perform transactions? #2718
• Closes .returning() is not supported by mysql and will not have any effect. #2989

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