INTMDB-904: HELP: mongodbatlas_org_invitation is missing ORG_BILLING_READ_ONLY role support

[andreaangiolillo]

Description

Ticket: INTMDB-904
Link to any related issue(s): #1280

This PR removes the hardcoded validation on the roles field. We will rely on the API from now on.

Type of change:

• 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.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[martinstibbe]

LGTM

[jwilliams-mongo]

@andreaangiolillo LGTM with a few suggestions to use current doc links