[#182] Fix Asciidoctor warnings in the OpenIG guides

[vharseko]

Fixes #182

Changes

• partials/sec-release-levels.adoc: removed the [appendix], [#appendix-interface-stability], == Release Levels and Interface Stability heading and the intro paragraph. They repeated the heading of reference/appendix-interface-stability.adoc, which includes this partial. On the site the converter adds :leveloffset: -1 to the page, so the repeated == became a level-0 section (level 0 sections can only be used when doctype is book), and its id was assigned twice (id assigned to section already in use: appendix-interface-stability). The rendered page also had an empty <h2 id="appendix-interface-stability">; the PDF book had the appendix heading nested in itself. The heading and intro stay on the including page.
• reference/handlers-conf.adoc: the nested [open] + ==== block of the Router openApiValidation properties (added in OpenAPI/Swagger routes loader and request/response validator #150/Add OpenApiMockResponseHandler — spec-driven mock API with realistic test data #155) was never closed — the enclosing -- came right after it (unterminated open block). Added the closing ==== after mockMode.

The releaseversion: 5.2.4 in the component descriptor, also mentioned in the issue, lives in doc.openidentityplatform.org and is not changed here.

Verification

• asciidoctor.js over reference/index.adoc and gateway-guide/index.adoc: before — unterminated example block at handlers-conf.adoc:780/781; after — no messages.
• Antora 3.1.15 build of doc.openidentityplatform.org (ROOT + openig components) with the same changes applied to the converted pages: the three messages from the issue are gone and no new ones appear. The appendix page renders without the empty heading; the openApiValidation options render as two open blocks with mockMode inside.
• Not in scope: the 10 skipping reference to missing attribute warnings (${true}, ${userid}, …) in expressions/filters/handlers/chap-auditing — the text renders as written.