* 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
* added instructions for working on Insiders
also added a note about how to prevent installing things outside a venv,
which I hope will be useful to people
* fixed broken link
* incorporated changes suggested by @squidfunk
- clarified language about access to Insiders
- let's not suggest SSH access
- better heading for note on getting pip to work in venv only
* added annotation that mentions cloning with ssh
* fixed indent
* added instructions for checking out specific releases and also moved the bit about $GH_TOKEN to a place above
the pip install instructions
* separated out update instructions, revised git instructions
* bumped version number that insiders is based on
* removed --upgrade from update command
* made insiders changelog a section
* changed instructions for upgrading via git
