docs: Add syntax highlighting languages to code blocks.

Signed-off-by: Anders Kaseorg <anders@zulip.com>
(cherry picked from commit b29b6f6526)

tools/linter_lib/custom_check.py

@@ -98,6 +98,9 @@ markdown_whitespace_rules: List["Rule"] = [
         "pattern": "^#+[A-Za-z0-9]",
         "strip": "\n",
         "description": "Missing space after # in heading",
+        "exclude_line": {
+            ("docs/subsystems/hotspots.md", "#hotspot_new_hotspot_name_icon {"),
+        },
         "good_lines": ["### some heading", "# another heading"],
         "bad_lines": ["###some heading", "#another heading"],
     },