docs: Add all md files in root dir to linkchecker

[p-offtermatt]

Description

• Fixes links that are currently broken
• Enables the markdown checker on more files (all files in the repo root and in the docs/docs folder)

Closes: N/A

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Author Checklist

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I have...

• included the correct docs: prefix in the PR title
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• provided a link to the relevant issue or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

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I have...

• Confirmed the correct docs: prefix in the PR title
• Confirmed all author checklist items have been addressed
• Confirmed that this PR only changes documentation
• Reviewed content for consistency
• Reviewed content for spelling and grammar
• Tested instructions (if applicable)
• Checked that the documentation website can be built and deployed successfully (run make build-docs)

[MSalopek]

LGTM