doc: Update doc for MongoDB::Atlas::FlexCluster CFN resource

[jromo-mdb]

Proposed changes

• Added detailed step-by-step setup instructions for activating and using the FlexCluster resource
• Updated resource description with important deprecation notice directing users to use MongoDB::Atlas::Cluster instead
• Fixed documentation link to point to the README instead of the JSON template

Jira ticket EPIC: CLOUDP-#286685
Jira ticket DOCSP: DOCSP-53418

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

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
• 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

[Copilot]

Pull Request Overview

This pull request updates documentation for the MongoDB Atlas FlexCluster CloudFormation resource. The changes add comprehensive setup instructions and important deprecation notices to guide users toward the recommended resource.

• Added detailed step-by-step setup instructions for activating and using the FlexCluster resource
• Updated resource description with important deprecation notice directing users to use MongoDB::Atlas::Cluster instead
• Fixed documentation link to point to the README instead of the JSON template

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
examples/flex-cluster/README.md	New comprehensive setup guide with activation steps and template usage instructions
cfn-resources/flex-cluster/README.md	Enhanced description with deprecation notice and corrected documentation link

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[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

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[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

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[lantoli]

all PR checks should pass before merging, note that "pull-request-lint / Validate PR title (pull_request_target)" is failing, you need to start the PR title with "doc:", also no need to have the ticket id, e.g. title:

doc: Update doc for MongoDB::Atlas::FlexCluster CFN resource

[lantoli]

LGTM , good to merge after addressing the comments and make sure all PR checks are passing. thanks!

[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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