[Docs] Kedro docs should validate internal links and build `--strict`

[stichbury]

Description

The docs should have a validation section in mkdocs.yml to warn of missing files and broken links. Let's add this as per the MkDocs configuration docs. The goal is to fix all those warnings.

If you then use build --strict flag (mkdocs build --strict) in your CI builds, you'll catch missed warnings since the docs won't build, and then get them fixed before publication. It's a horrible experience to have broken links, even if it is understandable after the revised information architecture of late.

This is what we use for Vizro:

validation:
  omitted_files: warn
  absolute_links: warn
  unrecognized_links: warn
  anchors: warn

Documentation page (if applicable)

Context