Added documentation on how deploy mixin with terraform

[wilfriedroset]

What this PR does

This PR introduces a new documentation about how to deploy mixin in Grafana and Mimir with terraform.

Full Disclosure: I'm working at OVHcloud and we are the maintainers of both ovh/mimirtool and ovh/mixtool providers 🙏🏾

Which issue(s) this PR fixes or relates to

Checklist

• [ ] Tests updated
• [x] Documentation added
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]