docs(datasource/aws-rds): improve readme

[HonkingGoose]

Changes

• Improve the aws-rds datasource readme:
  • Warn that the default datasource config does not work with Amazon Aurora
  • Link to docs for Amazon Aurora
  • Convert bold headings to actual headings
  • Put descriptions of codeblock in the codeblocks
  • Show user's configuration for "terraform + rds datasource + aurora-mysql"

Context

A user showed their config for "terraform + rds datasource + aurora-mysql", that should probably go in the docs.

They also said that the docs don't warn that by default the aws-rds datasource does not work with Amazon Aurora. So I'm warning about that in the docs too now.

Drive-by PR, not closing any issue with this, but it does resolve a bit of the discussion:

• 19928

Documentation (please check one with an [x])

• [x] I have updated the documentation, or
• [ ] No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• [x] Code inspection only, or
• [ ] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [ ] Both unit tests + ran on a real repository