Under construction

[beforan]

This PR adds:

• a PR template to help enure metadata is added correctly to new doc pages
• a bit of landing page tidy up
• document type metadata to all existing content docs
• an under construction component for placeholder pages where content is not ready but the route should exist (e.g. to prevent broken links)
• landing page links all go to valid routes with under construction placeholders
• journey pages and landing page content placeholders are at placeholder (NOT FINAL) routes. as docs content comes in, the placeholders should move and the links be updated!
• placeholder pages excluded from sidebar nav
  • the nav structure is subject to change, but this still ensures only actual content appears in the sidebar, even if there are placeholders for links

Sorry it's a lot of files but it's mostly creating placeholders at landing page linked routes, or just adding docType metadata to existing docs.

[AndyRae]

Great idea