Fix links to "old" documentation and mention the new one in the README

[skip-ci]

Author: kocsismate

README.md

@@ -131,8 +131,9 @@ contribute:
 
 - [Contributing to PHP](/CONTRIBUTING.md)
 - [PHP coding standards](/CODING_STANDARDS.md)
-- [Mailing list rules](/docs/mailinglist-rules.md)
-- [PHP release process](/docs/release-process.md)
+- [Internal documentation](https://php.github.io/php-src/)
+- [Mailing list rules](/docs-old/mailinglist-rules.md)
+- [PHP release process](/docs-old/release-process.md)
 
 ## Credits