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/load.go

@@ -184,6 +184,11 @@ func assemble(opts Options) (*site, []Problem, error) {
 		return nil, nil, err
 	}
 	problems = append(problems, sp...)
+	cp, err := s.checkContent()
+	if err != nil {
+		return nil, nil, err
+	}
+	problems = append(problems, cp...)
 	if len(problems) == 0 {
 		rp, err := s.render()
 		if err != nil {