Issue: SPR-16040

Cross-reference sections on HttpMessageReader/Writer and
HttpMessageConverter and improve the content.

Issue: SPR-16260

Issue: SPR-16240

A new HTTP/2 support section has been added in the Spring MVC
documentation. Also, a dedicated wiki page is created for specific
container support information.

Issue: SPR-15821

Issue: SPR-16087

Issue: SPR-16074

Issue: SPR-16041

Easier to find (at the top level) and better summary of testing support.
Improve structure headings names.
Remove or update outdated content.

Optimized for viewing in left TOC.

Issue: SPR-16024

Issue: SPR-15994

Pending -- WebSocket, WebTestClient, more details around annotation
processing, exception handling, and view resolution.

Issue: SPR-15149, SPR-16009

Issue: SPR-15149

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

Issue: SPR-15149

Issue: SPR-15872

Issue: SPR-15149

Refer to both Servlet and reactive stacks in web.adoc
Move web-integration.adoc to the very end
Link in web-flux-functional.adoc
Insert "Spring Web MVC" or "Servlet" prefix in chapter titles

Issue: SPR-15149

Issue: SPR-15643

ContentNegotiationManagerFactoryBean now provides an option to
explicitly set the strategies to use vs customizing a fixed
list of default strategies.

Issue: SPR-11114

Closes gh-1461

Issue: SPR-15612

Issue: SPR-15465

Issue: SPR-15211

Issue: SPR-15365

Issue: SPR-14997

This commit removes docbook from the documentation toolchain and
instead makes use of asciidoctor to render the reference documentation
in HTML and PDF formats.

The main Gradle build has been refactored with the documentation tasks
and sniffer tasks extracted to their own gradle file in the "gradle"
folder.

A new asciidoctor Spring theme is also used to render the HTML5 backend.

Issue: SPR-14997

Issue: SPR-15258

Issue: SPR-12674

Issue: SPR-15155