Proposal: Experimental Rulechecks

[daghack]

resolves #5176

Implements the feature and updates the CopyIgnoredFile rule check to be experimental through the new proposal.

[dvdksn]

I also think we should add something to the _index.md listing page to indicate which checks are experimental

buildkit/frontend/dockerfile/linter/docs/_index.md

Lines 32 to 35 in 27efea4

	<tr>
	<td><a href="./{{ .PageName }}/">{{ .Name }}</a></td>
	<td>{{ .Description }}</td>
	</tr>

[dvdksn]

LGTM!

[tonistiigi]

This looks good, but we should still make sure we can remove the experimental opt-in from the CopyIgnoredFile rule with the next release.