docs: adopt NuxtLink for internal routes

[wingkwong]

Change Summary

Currently if you directly access a page then click a link, the target link will not be correct. This PR is to change <a href="...">...</a> to <NuxtLink/> with full path for all internal routes. External routes will remain unchanged.

• closes: Link in REST APIs documentation redirects to a 404 page #5468

Change type

• feat: (new feature for the user, not a new feature for build script)
• fix: (bug fix for the user, not a fix to a build script)
• docs: (changes to the documentation)
• style: (formatting, missing semi colons, etc; no production code change)
• refactor: (refactoring production code, eg. renaming a variable)
• test: (adding missing tests, refactoring tests; no production code change)
• chore: (updating grunt tasks etc; no production code change)