Migrate docs to antora

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

spring-shell-docs/modules/ROOT/pages/options/basics/annotation.adoc

@@ -0,0 +1,13 @@
+[[using-shell-options-basics-annotation]]
+= Annotation
+:page-section-summary-toc: 1
+
+ifndef::snippets[:snippets: ../../../../../src/test/java/org/springframework/shell/docs]
+
+`Option` annotation can be used to define an option name if you
+don't want it to be same as argument name.
+
+[source, java, indent=0]
+----
+include::{snippets}/OptionSnippets.java[tag=option-with-option-annotation]
+----

spring-shell-docs/modules/ROOT/pages/options/basics/index.adoc

@@ -0,0 +1,12 @@
+[[using-shell-options-basics]]
+= Basics
+:page-section-summary-toc: 1
+
+ifndef::snippets[:snippets: ../../../../../src/test/java/org/springframework/shell/docs]
+
+This section gives a generic idea how an option can be defined. Following
+sections, beyond basics, discuss more about how various option behaviour
+can be accomplished for a particular use case.
+
+
+

spring-shell-docs/modules/ROOT/pages/options/basics/legacyannotation.adoc

@@ -0,0 +1,28 @@
+[[using-shell-options-basics-legacyannotation]]
+= Legacy Annotation
+
+ifndef::snippets[:snippets: ../../../../../src/test/java/org/springframework/shell/docs]
+
+Having a target method with argument is automatically registered with a matching
+argument name.
+
+[source, java, indent=0]
+----
+include::{snippets}/OptionSnippets.java[tag=option-without-annotation]
+----
+
+`@ShellOption` annotation can be used to define an option name if you
+don't want it to be same as argument name.
+
+[source, java, indent=0]
+----
+include::{snippets}/OptionSnippets.java[tag=option-with-annotation]
+----
+
+If option name is defined without prefix, either `-` or `--`, it is discovered
+from _ShellMethod#prefix_.
+
+[source, java, indent=0]
+----
+include::{snippets}/OptionSnippets.java[tag=option-with-annotation-without-prefix]
+----

spring-shell-docs/modules/ROOT/pages/options/basics/programmatic.adoc

@@ -0,0 +1,19 @@
+[[using-shell-options-basics-registration]]
+[[using-shell-options-basics-programmatic]]
+= Programmatic
+:page-section-summary-toc: 1
+
+ifndef::snippets[:snippets: ../../../../../src/test/java/org/springframework/shell/docs]
+
+Programmatic way with `CommandRegistration` is to use `withOption` to define
+an option.
+
+[source, java, indent=0]
+----
+include::{snippets}/OptionSnippets.java[tag=option-registration-longarg]
+----
+
+`CommandRegistration` can be defined as a bean or manually registered
+with a `CommandCatalog`.
+
+NOTE: Check below sections for other option types, i.e. short format.