feature: Allow turning on autoexec in mkdocs.yml

Is your feature request related to a problem? Please describe.

I'd like to be able to MARKDOWN_AUTO_EXEC from a configuration file rather than an environment variable. This is in order to make it easier for a new contributor to a project to naively run mkdocs build without issue, or generally to have all of the build config in files rather than partially in environment variables.

Describe the solution you'd like

When using markdown_exec as a mkdocs plugin there is an option to turn on autoexec from mkdocs.yml

Describe alternatives you've considered

Always setting the environment variable. Will work, but is one extra step and introduces potential for errors when forgetting to do so.

Additional context