[docs] Migrate CSS_Coding_Style

[sarjona]

The URL of the page on https://docs.moodle.org/dev/

https://docs.moodle.org/dev/CSS_Coding_Style

Is this documentation specific to a Moodle version?

No

What location would you suggest in the new docs?

/docs/guides/styles or /general/development/policies/codingstyle/css

Related with https://github.com/moodle/devdocs/issues/983