Organize Legacy Snapshot docs (#26)

* Rename & lint 'announcements'

* Migrate 'Overview'

* Move 'architecture'

* Migrate 'Components' page

* Create categories for components

* Move core components

* Move Deck's sub-docs

* Move 'Tide' page & its subpages

* Move 'Optional Components'

* Move 'CLI Tools'

* Move 'Pod Utilities'

* Create placeholder pages for undocu'ed components

* Move 'Deprecated Components'

* Move Plugins & External Plugins

* Move 'Deck' page

* Rename 'Contribution Guidelines' filename

* Rename & Move other files

* Apply suggestions from code review

* Rename restcoverage.md

* Add 'Testing Prow' placeholder page

* Move 'fallbackcheck.md' and 'fakegitserver.md'

* Add links to code directories