Don't show 404 if references are not in place

[tuxmea]

Short description

• add dummy content to all generated documentation files
• remove resource types which have been removed from puppet 6

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[miharp]

1. Seems like maintaining all these stubs could be a pain, consider a custom 404.html instead of per-page stubs
   One file at the repo root handles every missing URL via GitHub Pages' built-in 404 routing, instead of committing a bunch of stub files. Users get a clear explanation and links back to working sections. The trade-off is it relies on GitHub Pages' routing, and isn't visible locally with jekyll serve.
2. If keeping the stubs, improving the messaging would help — e.g. a styled callout using the existing alert.html include (type warning) rather than plain text, so it looks intentional rather than broken.
3. Canonical URL bug in the facter stubs
   Both _openfact_5x/cli.md and _openfact_5x/core_facts.md have canonical: "/openvox/latest/..." — should be /openfact/latest/....
4. Stubs for removed resource types are misleading
   Types like interface, router, vlan, and whit were removed from Puppet 6+. If they don't exist in OpenVox 8, stub pages for them imply they do. The nav entries for those types should be removed instead.
5. Nav not updated to match removed types

• The data/nav/openvox_8x.yml still lists 37 resource type entries that have no corresponding stub file and don't exist in the OpenVox source (lib/puppet/type/). These would all 404 after this PR merges. Verified against https://github.com/OpenVoxProject/openvox — OpenVox 8 only ships these core types: exec, file, filebucket, group, notify, package, resources, schedule, service, stage, tidy, user. Everything else (augeas, cron, host, mount, all nagios*, yumrepo, zfs, zone, zpool, etc.) was moved to separate modules in Puppet 6+ and shouldn't be in the core type reference nav at all.

6. Orphan stub

• types/overview.md has a stub but no nav entry — dead page.