SpringBoot Link all three document formats

Per issue 26314, add links to each document format (multi-page HTML, single-page HTML, and PDF) such that each document format links to the other two.

Comment From: wilkinsona

Thanks for the PR, @buzzardo, but we already cross-link all three formats. See https://docs.spring.io/spring-boot/docs/2.4.x/reference/htmlsingle/#boot-documentation-about, for example.