Validate internal links during build

[thomasrebele]

This PR imports the link validator from https://www.veriphor.com/articles/link-and-image-render-hooks/.

Currently many of the links are broken, so I set it to warning. The link validator said that no ref shortcuts should be used. So these would need to be updated first:

This render hook may be unable to resolve destinations created with the ref and relref shortcodes. Unless you set the error level to ignore you should not use either of these shortcodes in conjunction with this render hook.

The links created by the ref shortcuts are working. The link validator just prints a false warning. So we could merge the link validator, and remove the ref shortcuts in a separate PR.

Comparison:

• using ref
  • resolves pages in the whole site (e.g., {{< ref "hive-on-tez" >}} resolves to development/desingdocs/hive-on-tez/)
  • prints error if a page could not be found
  • does not check anchor links (e.g., {{< ref "#configuring-hive" >}} / "Configuring hive" in the configuration properties, is broken, however, there is no warning)
• using normal markdown links with Veriphor link validation
  • seems to require absolute path
  • can be configured if broken links are to be ignored or whether a warning or error should be reported
  • checks anchor links (with a limitation)
  • we can avoid the warnings related to ref links by replacing {{< ref "..." >}} with {{% ref "..." %}}