[New Doc PR] - Document migration from 2.x to 3.x #4327

[dalexandrov]

Resolves "Document migration from 2.x to 3.x" #4327

[tjquinno]

Two very small suggestions.

[barchetta]

Didn't we remove a bunch of deprecated APIs in 3.x? Shouldn't those be listed in these guides?

[m0mus]

This guide is created for users who have Helidon 2.x application and want to upgrade to 3.x. Now it looks like a copy of Helidon 3.0 announcement article. It's not an announcement. It must contain step by step instructions about how to upgrade a project. It must contain:

1. JDK version upgrade. This document partically has it, but it only states that Java 17 is supported. You should say that new JDK must be installed and configured on your computer. Possibly make a link to prerequisites.adoc where explained how to set JAVA_HOME.
2. New versions of MicroProfile (5.0) and Jakarta EE APIs. Your document partially has it. Never say that we support Jakarta EE 9.1. It's wrong. We only support selected subset of Jakarta EE 9.1 APIs. Make sure that verison number is consistent. I see sometimes 9.0, sometimes 9.1. For each specification I miss the version number used in 2.x. You have only the new version number. It would be great to include a short list ofbackwards incompatible chages for each specification. Ideally with some samples.
3. Deprecations. I miss a full list of deprecated APIs and equivalest new APIs.
4. Other backwards incompatible changes if any.

fixed)

[ljamen]

Initial review comments - open questions re: formatting are for me (we need to come up with a standard and then I can go back and make the edits.)