Convert AsciiDoc documentation to Markdown

[romain-grecourt]

Description

Forward-port #12220 to main, converting AsciiDoc documentation to
Markdown.

The Markdown is flavored using HTML comment directives so the source still
renders as vanilla Markdown.

On main, the former Helidon SE content is published under the top-level
Modules and Guides sections. This forward-port also includes the reconciled
main-branch documentation updates.

The configuration documentation generator now outputs Markdown instead of
AsciiDoc, with simpler anchor handling for included pages. Generated
configuration docs are checked in under docs/config, and CI regenerates them
and fails if the checked-in output is out of date.

Aggregate Javadocs are generated from the dedicated javadocs/pom.xml module
instead of docs/pom.xml.

Documentation

This pull request is the main forward-port of the documentation conversion
in #12220. It publishes the former SE documentation as top-level Modules and
Guides, including generated Markdown configuration references.

Validation

Validation included:

• focused configuration-reference generator tests, repeated stable Config
  generation, and the CI freshness check for checked-in docs/config output;
• two byte-identical differential audit runs, with all 16 fixtures passing and
  zero actionable findings; and
• checkstyle, copyright, ShellCheck, and whitespace checks.

[tomas-langer]

Of the five review findings, these two are blockers and should be fixed as part of this PR before merge:

• Restore the aggregate Javadocs main artifact in the validation-to-release cache handoff.
• Correct the copyright notice in the newly added javadocs/pom.xml.

The remaining three findings can be addressed in a follow-up PR:

• Update literal snapshot versions when preparing versioned documentation.
• Update the obsolete generated-config URL in WebClientDiscovery Javadoc.
• Point the v27 Maven guide to main-line application-POM guidance.