Update the embedded Kroki extension to asciidoctor-kroki 2.0.0.rc.3 (#516) (#2111)

kroki-extension.rb is now the gem's version.rb, preprocess.rb, cache.rb and
extension.rb concatenated into one loadable script, with the plugin's three
local changes (header, "-intellij" referer suffix, registration block).

PlantUML !include, !includesub, !include_once, !include_many and !includeurl
are resolved before the diagram is sent to Kroki: relative to the diagram
file of the block macro, then to each kroki-plantuml-include-paths entry;
http(s) includes are fetched and inlined, only <stdlib> passes through. The
Kroki server otherwise drops a local include it cannot read and renders a
degraded diagram without warning. A failed diagram no longer aborts the
document, and block macro targets resolve relative to the document.

Tests decode the generated Kroki URL and assert the included text is inlined,
for a macro target and for an inline block using kroki-plantuml-include-paths.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

Author: cachescrubber

CHANGELOG.adoc

@@ -9,7 +9,7 @@ This document provides a high-level view of the changes introduced by release.
 
 === 0.45.7
 
-...
+- Update the embedded Kroki extension to asciidoctor-kroki 2.0.0.rc.3: PlantUML `!include`, `!includesub`, `!include_once`, `!include_many` and `!includeurl` directives (relative to the diagram file, plus `kroki-plantuml-include-paths`) are now resolved before the diagram is sent to Kroki, a failed diagram no longer aborts the page, and block macro targets resolve relative to the document (#516)
 
 === 0.45.6
 

doc/users-guide/modules/ROOT/pages/features/preview/diagrams.adoc

@@ -111,6 +111,18 @@ To avoid this, consider using `interactive` for Kroki diagrams as an alternative
 See <<interative-vs-inlined>> and <<avoid-flicker>> for a discussion of tradeoffs.
 ====
 
+=== PlantUML includes with Kroki
+
+The Kroki server has no access to the local file system, so the plugin resolves PlantUML
+`+!include+`, `+!includesub+`, `+!include_once+`, `+!include_many+` and `+!includeurl+` directives
+before the diagram is sent (plugin version 0.45.7+, using the include preprocessor of
+asciidoctor-kroki 2.0).
+Relative include paths resolve against the directory of the `.puml` file referenced by the block macro,
+then against each directory in the `kroki-plantuml-include-paths` attribute; `http(s)` includes are fetched
+and inlined as well, only `+<stdlib>+` includes are passed through to the server.
+Diagrams written inline in a `[plantuml]` block have no file location, so only absolute paths and the
+`kroki-plantuml-include-paths` directories work for their includes.
+
 == Using embedded Mermaid support
 
 As an experimental option in plugin version 0.38.1+, there's an embedded Mermaid support that is embedded in the plugin and will render Mermaid diagrams within the IDE without external helpers.