Add more information about devfile samples in devfile.io

[thepetk]

/area documentation

Which area is this documentation change related to?

/area registry

Issue details

The current documentation doesn't explain in depth the usage & purpose of devfile samples in the devfile registry.

We could create a new page that covers:

• What is a devfile sample?
• What is the difference of devfile samples and registry stacks?
• When devfile samples are used?
• Where the devfile samples are used?

This could help users that want to distinguish the two different concepts: stacks & samples

Acceptance Criteria

• The documentation now includes a new page that now covers the above topics