Migrate documentation to asciidoctor

This commit migrates to asciidoctor to ease writing documentation.

Issues: SPR-10960

CONTRIBUTING-DOCUMENTATION.adoc

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+= How to contribute to the reference
+
+The Spring reference now uses http://asciidoctor.org/[asciidoctor]. This document describes how to contribute documentation updates to the Spring reference.
+
+== Building with Gradle
+
+You can build the documentation using gradle using the `asciidoc` task. For example, from the project root execute the following command:
+
+  gradlew asciidoc
+
+the output will be available at `spring-framework/build/asciidoc/index.html`
+
+== Live editing
+
+One of the nice features about using asciidoctor is the support for live editing.
+
+You will find a Guardfile already present at `spring-framework/src/asciidoc/Guardfile`. Then ensure to follow the setup instructions within the http://asciidoctor.org/docs/editing-asciidoc-with-live-preview/[Editing AsciiDoc with Live Preview] document.
+
+When running `guard start` within the `src/asciidoc/` folder, any changes to the `src/asciidoc/index.adoc` file will automatically be written at `src/asciidoc/build/index.html`.
+
+== Documentation notes
+
+Some notes on documentation
+
+* It is important to keep whitespaces to a minimum to make it simple to edit files. This means use an editor with line wrapping rather than manually inserting hard returns.
+* http://asciidoctor.org/docs/asciidoc-syntax-quick-reference/[Asciidoctor Quick Reference]
+* http://asciidoctor.org/docs/user-manual/[Asciidoctor Manual]
+* http://asciidoctor.org/docs/asciidoc-writers-guide/[Asciidoctor Writers Guide]