Add user guides for external databases and operator

[hallaroo]

We need user guides to cover how to use external databases and operator to the Vitess website. Currently, there is information distributed around the website that should be consolidated and expanded upon.

[satyam0051]

I want to work on this so please assign to me

[kristee-planetscale]

Linking and closing the following issues as they should be done along with this issue:

Document: External DB RDS Document: External DB Aurora Document: External DB CloudSQL