Update, test documentation links

[mciantyre]

The PR updates documentation links to use intra-documentation links. The PR then introduces a CI job to make sure those links are valid.

[mciantyre]

bors try

[bors[bot]]

try

Build failed:

• doclinks

[mciantyre]

bors merge

[bors[bot]]

Build succeeded:

• features (rtic,rt)
• test
• clippy
• features
• features (rtic)
• features (systick)
• precompiled
• template
• xplat (macos-latest)
• xplat (ubuntu-latest)
• xplat (windows-latest)
• doc
• format
• features (systick,rt)
• features (usb-logging,rt)
• features (usb-logging)
• doclinks