docs: show child links on Expression Compatibility page (#4319)

The toctree on the Expression Compatibility index used `:hidden:`, leaving
the page with no visible way to navigate to its category sub-pages
(aggregate, array, cast, datetime, map, math, misc, string, struct).
Drop `:hidden:` and raise `:maxdepth:` to 2 so Sphinx renders the child
links inline.

Author: andygrove

docs/source/user-guide/latest/compatibility/expressions/index.md

@@ -26,8 +26,7 @@ the [Comet Supported Expressions Guide](../../expressions.md) for more informati
 Compatibility notes are grouped by expression category:
 
 ```{toctree}
-:maxdepth: 1
-:hidden:
+:maxdepth: 2
 
 aggregate
 array