4.x - Externalize and compile documentation java snippets

[romain-grecourt]

Description

Fixes #8293
Fixes #8313

Externalize and compile docs java snippets.
Introduce conventions and style for Java snippets across all docs (see below).

Documentation

This PR contains docs update only.

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Changes

• Added compiler:compile execution in docs to compile the snippets
• Fix snippets indentations (E.g. XML for dependencies)
• Fix multiple XML roots in snippets (wrap with a parent)
• Fix comments used for annotation in YAML snippets
• Fix IntelliJ warnings and errors to make them useful (now that the noise of the inlined Java snippets is removed)
• Fix typos reported by IntelliJ spell checking
• Break lines over 140 characters
• Use hocon as the language type for JSON snippets with annotation (syntax: // <1>)
• Normalize(-ish) the request/response snippets ; split out the JSON responses into dedicated code blocks
• Add the fit role to many images

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New conventions

• 1:1 between .adoc and .java (one "Snippets" class for each document)
• package layout mirrors src/main/asciidoc (base package io.helidon.docs)
• classes are named using the camel case name equivalent of the document + Snippets suffix (E.g. reactive-messaging.adoc -> ReactiveMessagingSnippets.java
• Snippets are named snippet_{index} where index is the snippet index in the document (starting at index 1)
• If the snippet is composed of statements, it is enclosed with a method void snippet_{index} {}
• If the snippet is a class (I.e class, record, interface, enum) ; and the name conflicts with any other snippet in the class, the snippet is enclosed in a class named Snippet{Index}
• If the snippet has unresolved references, they can be stubbed, with method parameters (if enclosed with a method), or by adding stubs at the top of the file (E.g. records, classes, constants etc.)
• Annotation comments (E.g. // <1>) are not padded ; (don't bother aligning them ; it is not something that we can consistently maintain) ; instead they should be separated by a single space character

Changes in snippets

• imports are removed from snippets, this simplifies the maintenance of the snippets (note that we should address this in the future and provide a way to see the imports of snippets in the docs)
• var keyword is preferred to improve readability unless the type matters for the understanding of the snippet

[tomas-langer]

This brings in a much needed validation of code snippets that will make it easier to keep documentation up to date for our future (and current) releases.
Thanks!

[barchetta]

Great work!