(DOCSP-46837) Adds an exclude to avoid overwriting handwritten atlas-api file

[sarahsimpers]

Proposed changes

This adds an exclude to the destination_files for copybara for the Atlas CLI docs repo so that the handwritten atlas-api file we are adding in the following PR is never overwritten by copybara: mongodb/docs-atlas-cli#838

Please note, I am having trouble running copybara locally at the moment so I am unable to test this, but this should prevent overwriting the file

Jira ticket:

https://jira.mongodb.org/browse/DOCSP-46837

Closes #[issue number]

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 in document requirements section listed in CONTRIBUTING.md (if appropriate)
• I have addressed the @mongodb/docs-cloud-team comments (if appropriate)
• I have updated test/README.md (if an e2e test has been added)
• I have run make fmt and formatted my code