Implement strategy to redirect older documentation links to the new site

[philwebb]

Any existing links to https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#<fragment> or https://docs.spring.io/spring-boot/docs/current/reference/html/<name>#<fragment> will break since we've migrated to Antora. Additionally our anchor rewrite rules will also fail.

We ideally need a strategy to help users find the new page. It probably needs to be client-side javascript, perhaps combined with some HTTPD rewrite rules.