This adds a dedicated migration guide for users moving from the Spring Security SAML Extension to the built-in SAML 2.0 support.
Includes:
- Content migrated from the project wiki
- xref links for `saml2Login`, `saml2Logout`, and `saml2Metadata`
- Metadata example moved to Examples Matrix
- Cleanup and naming per review feedback
Closes gh-11161
Signed-off-by: snowykte0426 <snowykte0426@naver.com>