Hide Skeleton docs

[gregtatum]

In issue #681, Zibi audited our docs. Skeleton must be exposed for the Data Provider machinery, but it's not really a public API that our users should directly use (unless we decide otherwise). I could make the skeleton mod pub(mod), and only re-export the Skeleton and SkeletonError types, but I figured it would be better to not even expose them to user-facing documentation.

This would be the same way that the pattern module is working.

[coveralls]

Pull Request Test Coverage Report for Build 77d84fda59d08c591bc1736ed5e36a60f5d1e21e-PR-689

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.009%) to 73.439%

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Files with Coverage Reduction	New Missed Lines	%
experimental/provider_ppucd/src/parse_ppucd.rs	1	93.0%

Totals
Change from base Build e78289331a514adbce008364653dce2fed9ddced:	-0.009%
Covered Lines:	8140
Relevant Lines:	11084

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💛 - Coveralls

[sffc]

I like #[doc(hidden)]. It's a nice way to get around the problem of needing to export types that aren't intended for public use.