Add caching of parsed completion documentation markdown to reduce flicker when selecting (#31546)

Related to #31460 and #28635.

Release Notes:

- Fixed redraw delay of documentation from language server completions
and added caching to reduce flicker when using arrow keys to change
selection.

Cargo.lock

@@ -9567,6 +9567,7 @@ dependencies = [
  "assets",
  "base64 0.22.1",
  "env_logger 0.11.8",
+ "futures 0.3.31",
  "gpui",
  "language",
  "languages",