Migrate docs to antora

- This is basically copy from main branch minus all
  terminal ui things.
- Relates #971

spring-shell-docs/modules/ROOT/pages/testing/basics.adoc

@@ -0,0 +1,22 @@
+[[using-shell-testing-basics]]
+= Basics
+
+ifndef::snippets[:snippets: ../../../../src/test/java/org/springframework/shell/docs]
+
+Spring Shell provides a number of utilities and annotations to help when testing your application.
+Test support is provided by two modules: `spring-shell-test` contains core items, and
+`spring-shell-test-autoconfigure` supports auto-configuration for tests.
+
+To test _interactive_ commands.
+
+[source, java, indent=0]
+----
+include::{snippets}/TestingSnippets.java[tag=testing-shelltest-interactive]
+----
+
+To test _non-interactive_ commands.
+
+[source, java, indent=0]
+----
+include::{snippets}/TestingSnippets.java[tag=testing-shelltest-noninteractive]
+----

spring-shell-docs/modules/ROOT/pages/testing/index.adoc

@@ -0,0 +1,20 @@
+[[using-shell-testing]]
+= Testing
+:page-section-summary-toc: 1
+
+ifndef::snippets[:snippets: ../../../../src/test/java/org/springframework/shell/docs]
+
+Testing cli application is difficult due to various reasons:
+
+- There are differences between OS's.
+- Within OS there may be different shell implementations in use.
+- What goes into a shell and comes out from a shell my be totally
+  different what you see in shell itself due to control characters.
+- Shell may feel syncronous but most likely it is not meaning when
+  someting is written into it, you can't assume next update in
+  in it is not final.
+
+NOTE: Testing support is currently under development and will be
+      unstable for various parts.
+
+

spring-shell-docs/modules/ROOT/pages/testing/settings.adoc

@@ -0,0 +1,24 @@
+[[using-shell-testing-settings]]
+= Settings
+
+ifndef::snippets[:snippets: ../../../../src/test/java/org/springframework/shell/docs]
+
+Built in emulation uses terminal width 80 and height 24 on default.
+Changing dimensions is useful if output would span into multiple
+lines and you don't want to handle those cases in a tests.
+
+These can be changed using properties `spring.shell.test.terminal-width`
+or `spring.shell.test.terminal-height`.
+
+[source, java, indent=0]
+----
+include::{snippets}/TestingSnippets.java[tag=testing-shelltest-dimensions-props]
+----
+
+`ShellTest` annotation have fields `terminalWidth` and `terminalHeight`
+which can also be used to change dimensions.
+
+[source, java, indent=0]
+----
+include::{snippets}/TestingSnippets.java[tag=testing-shelltest-dimensions-field]
+----