docs: expand CSS module import documentation

[bartlomieju]

Expands the with { type: "css" } documentation in the modules guide to
cover the details requested in #3257, which the existing short section
left out.

Adds the dynamic import(..., { with: { type: "css" } }) form, the
read-permission requirement for loading a stylesheet from disk, and a
description of the slice of the CSSStyleSheet interface that Deno
actually implements: cssRules as a frozen array of CSSRule (not a
live CSSRuleList), CSSRule.cssText, replace/replaceSync dropping
top-level @import rules, and the new CSSStyleSheet() constructor
without support for its options argument. Also spells out the
limitations that follow from Deno having no DOM: a sheet can't be adopted
anywhere, the rule list comes from a naive top-level split, and
insertRule/deleteRule are not implemented.

I verified each of these claims against the implementation in
ext/web/css_stylesheet.rs before documenting them.

Closes #3257.