feat(glossy): Remove hide/hidden modifier

This modifier was not orthogonal with other modifiers (for example, it
could not be used together with :noindex)
Added documentation and modified test to reflect that @term[] with an
empty supplement has exactly the same result *and* is more orthogonal.

This partially reverts commit 5957432d5d.

glossy/tests/hidden-terms/test.typ

@@ -24,9 +24,9 @@
 
 = Hiding terms
 
-Here's a hidden term: @html:hide
+Here's a hidden term: @html[]
 
-Here's another hidden term: @css:hidden
+Here's another hidden term: @css[]
 
 And here's a shown term: @tps