feat(11.1-02): check module identifiers in docs and READMEs

- go/parser index of every modules/ package and sub-package, with methods,
  fields, interface methods and promoted members
- code spans in docs pages, module READMEs and the root README fail
  Check and docs:build when the named identifier does not exist
- scripts/check-phase11.1.sh with preconditions, deps, docs, forbidden,
  go and a self-test that plants one violation per rule

internal/docsite/docsite.go

@@ -229,6 +229,21 @@ func writeOutputs(out string, files map[string][]byte) error {
 	return nil
 }
 
+// checkContent runs the accuracy checkers over every page and the root
+// README.md: module identifiers named in code spans.
+func (s *site) checkContent() ([]Problem, error) {
+	docs, err := s.parseDocs()
+	if err != nil {
+		return nil, err
+	}
+	idx, problems, err := buildIdentIndex(s.opts.Root)
+	if err != nil {
+		return nil, err
+	}
+	problems = append(problems, s.checkIdentifiers(idx, docs)...)
+	return problems, nil
+}
+
 func sortProblems(problems []Problem) {
 	slices.SortStableFunc(problems, func(a, b Problem) int {
 		return cmp.Or(strings.Compare(a.File, b.File), cmp.Compare(a.Line, b.Line))