support markdown alerts in documentation page

### Describe the issue

Markdown has a feature called alerts that are supported [natively](https://github.com/orgs/community/discussions/16925) in GitHub and provide an excellent viewing experience in our GitHub documentation.

However, our documentation page does not support these magic alerts and [results](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_appconfig-readme.html) in something ok, but not great (search for the raw `![Note]` or `![Important]`.

I think the simplest thing to do here is to just omit the line of `![Note]` if it shows up. It's no biggie either way :). I would love to continue using this feature in the GitHub docs and not have to worry about how it looks in our documentation site.

### Links

n/a

Start by locating the documentation-page rendering path that handles GitHub Markdown, then reproduce the AWS CDK documentation result using an alert such as `![Note]` or `![Important]`. Done means Markdown alerts no longer render as raw markers or otherwise degrade the documentation page, while ordinary documentation output remains unchanged.

Written by the indexing model from the issue text.