[docs] Create documentation for `editor_styles.css`

[andrewnicols]

What is the Moodle feature that needs documenting?

The existence of editor_styles.css in plugins, including information about where it can be placed, what it's for, etc.

Is this documentation specific to a Moodle version?

None

Are you able to provide a patch for this?

None

[hamidayubdev]

@andrewnicols could you do me a favor of assign it to me. I want to work on it