This commit restructures the existing content to separate out Overview
content into its own section, and also introduces more formal structure
where reference type content can be expanded over time.
Issue: SPR-16547
@EnableWebFlux bootstraps both annotated controllers and functional
endpoints, so we need to be more explicit about which parts of the
configuration apply to which.
Issue: SPR-16360
This change allows much more usable TOC for the reactive
stack and will make WebFlux documentation easier to
contribute thanks to a clearer split between both stacks.
Issue: SPR-15149