docs: document the minimum Coder version for each provider release

[matifali]

Sometimes the Coder provider and Coder server can have version compatibility issues. To address this we should document on the index page the minimum required Coder version for each coder terraform provider release.

[matifali]

Doing it in #259. I can convert the Note to a Warning if needed.

[matifali]

Not closing now as I would like us to automate this in future.