doc: Enhances README with additional MongoDB Atlas and AWS CloudFormation documentation links and resource support details

[EspenAlbert]

Proposed changes

Enhances README with additional MongoDB Atlas and AWS CloudFormation documentation links and resource support details

Link to any related issue(s): CLOUDP-307844

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
• 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.

Manual QA performed:

• cfn invoke for each of CRUDL/cfn test
• Updated resource in example
• Published to AWS private registry
• Used the template in example to create and update a stack in AWS
• Deleted stack to ensure resources are deleted
• Created multiple resources in same stack
• Validated in Atlas UI
• Included screenshots

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 checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• For CFN Resources: I have released by changes in the private registry and proved by change
  works in Atlas

Further comments

[oarbusi]

LGTM, some minor comments

[colm-quinn]

LGTM, thanks for the updates!

[themantissa]

Responded to questions and added a few suggestions to hopefully help us clear more up in the README. Please give me a slack ping when ready again as I'm woefully behind on email 😄

[themantissa]

Just a bit of explanation and clarity:

Suggested change

- We **only** support resources enabled in the [AWS Cloudformation Third Party registry](https://console.aws.amazon.com/cloudformation/home#/registry/public-extensions?visibility=PUBLIC&type=RESOURCE&category=THIRD_PARTY) (select "Third party" as `Provider` and search by "MongoDB::Atlas").

- In order to ensure the highest quality experience and support, we only support resources enabled via the [AWS Cloudformation Third Party registry](https://console.aws.amazon.com/cloudformation/home#/registry/public-extensions?visibility=PUBLIC&type=RESOURCE&category=THIRD_PARTY) (select "Third party" as `Provider` and search by "MongoDB::Atlas"). Any usage outside of this workflow is not supported or recommended.

[themantissa]

Also in reading over the rest of the README - I think we need a header (double #) on "Enabling" that further reiterates where to enable the resources, i.e. the 3rd party registry. And also to do so in each acct/region needed. We only have this in troubleshooting but a header on Enabling hopefully will keep them out of troubleshooting. We might also want to include an "Upgrade" header?

[EspenAlbert]

Changed in a86cd5d to refer to our existing Activate the MongoDB Atlas CloudFormation public extensions which is more complete. WDYT?

[EspenAlbert]

Regarding Upgrade, I added a new guide here: a147282

[themantissa]

Thank you for the quick and complete turn around!

[marcosuma]

Merging this PR on behalf on @EspenAlbert