Enable mdoc typecheking for micrositeExtraMdFiles

[calvellido]

Currently, if you have extra markdown files in your site, and use mdoc as the typechecking tool, while these files will be built as part of the site, the code snippets won't be analyzed, as stressed out in the PR including mdoc as new typechecking plugin.

One solution would be the one @rafaparadela is proposing, by moving the extra .md files to the input directory, then compile them, and finally move them to the Jekyll folder.

Another one could be to use the mdoc --include doc.md in a new or one of the existing tasks.