fix: Fixes value of `data_process_region` in `federated_database_instance` data sources with additional cleanup in documentation

[AgustinBettati]

Description

Link to any related issue(s): CLOUDP-237334 and https://github.com/mongodb/terraform-provider-mongodbatlas/issues/2013

Changes that are included:

• Fixes documentation of the resource, specifically 2 important attributes (storage_databases storage_stores) are defined as exported but in fact are arguments that can be used in the configuration.
• dataProcessRegion was not being used in any of our tests and not mentioned in examples, added the attribute within our basic test and added a small example.
• When adding the attribute in our basic test detected it was not being returned in our data sources due to an incorrect conditional.

Type of change:

• [x] Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• [ ] New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• [ ] This change requires a documentation update
• [x] Documentation fix/enhancement

Required Checklist:

• [x] I have signed the MongoDB CLA
• [x] I have read the contributing guides
• [x] I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• [x] I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• [x] I have added any necessary documentation (if appropriate)
• [x] I have run make fmt and formatted my code
• [x] If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.
• [x] 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

[AgustinBettati]

Test that is failing is due to OUT_OF_CAPACITY when creating a cluster for TestAccFederatedDatabaseInstance_atlasCluster. Relevant test TestAccFederatedDatabaseInstance_basic executed successfully.

[github-actions[bot]]

APIx bot: a message has been sent to Docs Slack channel

[AgustinBettati]

Pending points before merging:

• receive review from team, address any possible comments
• rerun test to make sure all tests are passing (once capacity issue is not present)

After merge:

• close https://github.com/mongodb/terraform-provider-mongodbatlas/issues/2013 as no further action is needed.
• close Jira ticket.

[lantoli]

will re-run tests on Fri