Clarify early on the different types of drifts

[sjourdan]

As reported on Discord by the user "ptaag": the different types of drifts that are returned by the driftctl CLI aren't easily explained on the docs: what is a missing resource? what is an unmanaged resource? etc.

We should clarify that as early as page 1 of the docs site so as to let users know immediately what they will get from the tool!

[sjourdan]

Let's add a new block on the first "welcome" documentation page, like an overview, so users know ASAP what they will get from the product.

Draft TBD

Here's a typical driftctl output to insert in 1st-page doc:

Found 79 resource(s)
 - 8% coverage
 - 7 resource(s) managed by Terraform
     - 1/7 resource(s) out of sync with Terraform state
 - 72 resource(s) not managed by Terraform
 - 0 resource(s) found in a Terraform state but missing on the cloud provider

[sjourdan]

@GeraldCrescione WDYT about syncing www content and documentation 1st-page to make it a coherent welcome page?

maybe also adapt the youtube link (old FOSDEM vid??) etc.