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/render.go

@@ -253,6 +253,13 @@ func (s *site) parsePage(md goldmark.Markdown, p *Page) (ast.Node, *pageContext)
 	return doc, pctx
 }
 
+// parseRaw parses a Markdown body with the renderer's parser and slug IDs
+// but no page context, so link destinations stay as written.
+func (s *site) parseRaw(md goldmark.Markdown, body []byte) ast.Node {
+	ctx := parser.NewContext(parser.WithIDs(newSlugIDs()))
+	return md.Parser().Parse(text.NewReader(body), parser.WithContext(ctx))
+}
+
 // renderPage parses and renders a page body to HTML.
 func (s *site) renderPage(md goldmark.Markdown, p *Page) (renderedPage, error) {
 	doc, pctx := s.parsePage(md, p)