* Add `"relative_to_docs"` value to `absolute_links` enum
* refactor to use `oneOf`
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Co-authored-by: Jake Beazley <jake.beazley@johnlewis.co.uk>
* added missing "unrecognized_links"
* changed "definitions" to "$defs"
"$defs" is where people would expect definitions of
reusable subschemas to live
* 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
