implement ACL policy grants

[mariux]

thank you for your contribution.. the team will look into it asap.. ;) looks very good on first sight 👍

[zied-elouaer]

A few missing things:

• Document the new variable grants
• Add the new variable to the unit tests
• Make sure README.md is generated from README.tfdoc.hcl using terradoc.

[treksler]

i gave it a shot to address the missing things

• wasn't really sure what to add to the unit tests. I just added a grants block to one of the examples
• hopefully the documentation changes are what you require

[treksler]

the only errors in https://github.com/mineiros-io/terraform-aws-s3-bucket/actions/runs/3198717372/jobs/5336038710 seem to be around "dead" links, which aren't actually dead