As the mkdocs project does not publish its own JSON Schema for its
configuration, you can use the one we create from mkdocs.schema.json.
We are fully aware that the schema also depends on plugins so it will never be very comprehensive. However, it is a good starting point for spotting errors, providing autocompletion or documentation tooltips. Feel free to contribute your improvements.
If you are using vscode with yaml extension, the schema validation functionality will be automatically enabled.