2.2.x has no htmlsingle documentation variant anymore.

[larsgrefer]

Since version 2.2.x, the spring boot documentation has no htmlsingle variant anymore.

This will cause all links across the web pointing to https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/ to break, once 2.2.0 goes GA.

I propose to put a placeholder index.html under reference/htmlsingle/ which either redirects to the new documentation or shows at least a nice information about the fact that the documentation has changed (instead of a basic 404 error page)

[jnizet]

Losing the htmlsingle would be really sad. It's extremely useful to be able to Ctrl-F in the complete documentation to find what we want.