fix(11.1-07): check go fences, README src= and shell fences from the AST

- goLang follows the highlighter's chroma lookup, so golang and main.go need src=
- a src= fence in a module README is refused and never captioned
- shell fences inside callouts and lists are command-checked
- a fence with four or more leading spaces is an indented code block

internal/docsite/render.go

@@ -469,7 +469,9 @@ func scanFences(lines []string) []fence {
 
 func openFence(line string) (fence, bool) {
 	trimmed := strings.TrimLeft(line, " ")
-	if len(trimmed) < 3 || (trimmed[0] != '`' && trimmed[0] != '~') {
+	// CommonMark: a fence has at most three leading spaces. Four or more
+	// is an indented code block, which is what goldmark parses.
+	if len(line)-len(trimmed) > 3 || len(trimmed) < 3 || (trimmed[0] != '`' && trimmed[0] != '~') {
 		return fence{}, false
 	}
 	c := trimmed[0]