docs(dev): rewrite 15.8 Web App plugin guide to match implementation

[marevol]

Summary

The 15.8 Web App plugin developer guide (dev/webapp-plugin.rst) described an
Action + JSP page model that does not match any real fess-webapp-* plugin and
whose code examples do not compile. This PR rewrites the guide to document how
Web App plugins actually extend Fess.

Problems with the previous content

• JSP views cannot be served from a plugin. Plugin JARs are mounted onto the
  web application classpath (WEB-INF/classes), not WEB-INF/view, so the
  documented src/main/webapp/WEB-INF/view/.../index.jsp layout is never packaged
  or resolved.
• path_Example_IndexJsp does not exist for plugins. The FessHtmlPath
  constants are generated (FreeGen) only from core Fess views, so the page Action
  example does not compile.
• The API example does not compile. ApiResult.ApiResponse has no
  .result(Map...) method (the builder order was also reversed), and the base
  action rejects all requests unless access control is overridden.

What the guide now documents

Web App plugins extend Fess by adding or replacing DI (Lasta Di) components:

• Adding new components via app++.xml
• Overriding core components via fess+<componentName>.xml
• Adding REST API endpoints via fess_api++.xml + WebApiManager / BaseApiManager
• The required Fess-WebAppJar: true JAR manifest entry
• Build and installation, and how to customize the search screen (themes)
• A list of the published fess-webapp-* plugins

All examples are grounded in the real fess-webapp-example, fess-webapp-v1-api,
and fess-webapp-classic-api plugins.

Languages

Japanese (source of truth) was rewritten and verified against the implementation;
en, de, fr, es, ko, and zh-cn were re-translated to match.

Verification

• Code identifiers, values, file names, and paths are byte-identical across all
  seven languages.
• RST structure (directives, code blocks, list tables, :doc: references) is
  consistent across languages, and heading underline lengths are adequate
  (East-Asian-width aware).
• de/fr/es use correct orthography with full diacritics.