* added support for two new entries in mkdocs.yml
`exclude_docs` and `not_in_nav` were introduced in MkDocs Version 1.5
* added support for the validation field
* using $defs, supporting simplified form, no additional props
- this should support the forms of definitions described in the manual
https://www.mkdocs.org/user-guide/configuration/#validation
including uses with `nav` and `link` sections as well as without
- making use of a `$defs` section to avoid too much duplication
- added `"additionalProperties": false`
* moved `additionalProperties` for consistency with style
