docs: Standardize all IndieAuth spec references to W3C URL

- Updated 42 references from indieauth.spec.indieweb.org to www.w3.org/TR/indieauth
- Ensures consistency across all documentation
- Points to the authoritative W3C specification
- No functional changes, documentation update only

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

docs/examples/identity-page-customization-guide.md

@@ -328,7 +328,7 @@ Once your identity page is working:
 
 - **IndieWeb Chat**: https://indieweb.org/discuss
 - **StarPunk Issues**: [GitHub repository]
-- **IndieAuth Spec**: https://indieauth.spec.indieweb.org/
+- **IndieAuth Spec**: https://www.w3.org/TR/indieauth/
 - **Microformats Wiki**: http://microformats.org/
 
 Remember: The simplest solution is often the best. Don't add complexity unless you need it.