feat(guide): Add guide on how to test a VS Code extension

[benoitf]

What does this PR do?

Add a guide to try a VS Code extension in Eclipse Che without forking/building plug-in registry

What issues does this PR fix or reference?

eclipse-che/che#15982
https://issues.redhat.com/browse/RHDEVDOCS-1637

Specify the version of the product this PR applies to.

che 7+

[rkratky]

@benoitf, we fixed some lang. (thanks, @boczkowska) and markup issues, and it's ready for merging. Only thing I'm unsure of is the placement in guides. I see you pbbly wanted this in the End-user Guide under Using developer workspaces.

I originally thought it was meant for the Admin Guide because under Customizing devfile and plug-in registries, there's already a Using a VS Code extension in Che. So, I thought the new Testing a VS Code extension in Che belonged with it.

For the time being, I'm leaving it in the Admin Guide, but if you think it would be better in the End-user Guide, I suggest we move both of the VS Code extension topics there.

[benoitf]

Hello @rkratky
This is really something for end users and not admins
As here, any user can do that without admin access as it's just a way of providing custom devfiles

[benoitf]

But for example, customizing plugin registry or devfile registry is more an admin stuff so I'm unsure we can move everything