CORE-332 Document RDS, Neptune, Document DB

[ellisonc]

Description

Fixes #389. Adds documentation for RDS db resource type and adds support for Config file filtering for DBInstance resources

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.
• Attention Grunts - if this PR adds support for a new resource, ensure the nuke_sandbox and nuke_phxdevops jobs in .circleci/config.yml have been updated with appropriate exclusions (either directly in the job or via the .circleci/nuke_config.yml file) to prevent nuking IAM roles, groups, resources, etc that are important for the test accounts.

Release Notes (draft)

Added DBInstance resource type support to config files.

[zackproser]

LGTM!

[ellisonc]

re-running tests after enabling Macie in us-east-1, not sure how it got un-enabled but I want to keep the tests passing