Add 'examples' as a category into the registry

[svrnm]

As discussed during the SIG adding examples/tutorials to the registry seems like a good experiment: this would allow new-comers to OTel to explore different ways into getting started, also people creating those examples/tutorials can make them a little bit more visibile.

A requirement would be that those examples are vendor-agnostic.

[chalin]

I'd like to point out that with work on #770, we'll likely be proposing a way to offer links to examples from each of the SIG's website landing pages (under /docs/<SIG>).

[rquedas]

I agree with @chalin, having tutorials and examples within each SIG's website will make it easier for people to find them.

I also believe there is a difference between example and tutorial. I would look at the opentelemetry-collector-contrib repo as a great source of examples, while a tutorial would be focused in guiding me through process of creating one of those examples.