doc: DOCSP-53434 -- Update doc for CfnCluster CDK resource

[xargom]

Proposed changes

Jira ticket: DOCSP-53434

Please include a summary of the fix/feature/change, including any relevant motivation and context.

Added a link referring to the new flex clusters in the readme file.

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as
  expected)
• This change requires a documentation update

Required Checklist:

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• I have tested the CDK constructor in a CFN stack. See TESTING.md
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[oarbusi]

LGTM, we have to wait to merge this until flex cluster is fully released in CFN, right @lantoli ?

[antellezr-mdb]

LGTM, with a minor observation.

[lantoli]

LGTM, we have to wait to merge this until flex cluster is fully released in CFN, right @lantoli ?

@oarbusi already done

[lantoli]

LGTM once comments are addressed

[xargom]

Comments addressed

[lantoli]

LGTM, feel free to merge once the last comments are addressed