[docs] add machine-to-machine docs page

[hsluoyz]

See: https://www.reddit.com/r/webdev/comments/14peldv/help_needed_is_there_a_product_that_provides_the/

Add "machine-to-machine" and how to authenticate to the API to the existing docs: https://casdoor.org/docs/basic/public-api

[casbin-bot]

@leo220yuyaodog @Selflocking