fix: href does not exist, correct is path

[mcanouil]

This PR fixes the documentation which states the option to add file or external links is href when it is path (or at least the latter works as most users expect it to work).

This is from at least two feedback this week:

• Support YAML file with list of files for listing contents. quarto-cli#4903 (comment)
• How to link a external URL to a Post in Listings Section. quarto-cli#6338
• Redirect a blog post to an external link (while having the blog card) quarto-cli#6309

CC @dragonstyle