Issue: SPR-15973

Issue: SPR-15793

Issue: SPR-15266

Also marks @Conditional as @Documented, aligned with other annotations.

Issue: SPR-15266

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-15354

See gh-1349

Issue: SPR-14711

Issue: SPR-15154

Issue: SPR-15153

Issue: SPR-15072

Issue: SPR-14812

Issue:  SPR-14797

Issue: SPR-14765

Closes gh-1172

Issue: SPR-14594

Issue: SPR-14600

Closes gh-1110

Closes gh-1107

Closes gh-1100

See gh-1100

Closes gh-1099

This commit also removes the corresponding deprecated Servlet MVC variant and updates DispatcherServlet.properties to point to RequestMappingHandlerMapping/Adapter by default.

Issue: SPR-14129

Issue: SPR-14402

Closes gh-967

Closes gh-1089

See gh-1089

Closes gh-1092

Issue: SPR-14355

Issue: SPR-13994

Issue: SPR-13994

Issue: SPR-13994

Issue: SPR-14332

Issue: SPR-14087
Issue: SPR-14272
Issue: SPR-13535
Issue: SPR-13843
Issue: SPR-14164
Issue: SPR-14167