Improve documentation for the preview release

Right now the generated documentation for Terraform Registry is quite shallow - it only documents the schema, but doesn't provide any additional commentary on the purpose of different resources.

We don't need to write the commentary from scratch as we can mostly use what was already written for other parts of documentation, so we can have short introductions with links to the main documentation.

Acceptance criteria:

[ ] Introduction into Ably Terraform Provider for the index page (similar to the intro on https://ably.com/docs/control-api)
[ ] For resources that have some description in other Ably docs, have an introduction about them and link "learn more in Ably documentation" to relevant section of the main documentation.

┆Issue is synchronized with this Jira Task by Unito

ably / terraform-provider-ably

Improve documentation for the preview release #80