[#1129] Point the guides' Javadoc links at the published apidocs, and resolve {opendj-version} in the uninstall and monitoring pages

[vharseko]

Fixes #1129

Problem

• Javadoc links. The guides link to ../javadoc/index.html (and ../javadoc/index.html?org/…/X.html), which is 404 on the site: the Javadoc is published at https://doc.openidentityplatform.org/opendj/apidocs/. That Javadoc is the frameless kind, so the index.html?<class page> form would only open the overview there, not the class.
• {opendj-version}. The asciidoc-pre-process goal of doc-maven-plugin (AsciidocPreProcessMojo.updateVersionAttributes) resolves the version only by rewriting a :opendj-version: x.y.z line on the page itself. install-guide/chap-uninstall.adoc has no such line, so msiexec /x opendj-{opendj-version}.msi /quiet stays literal. admin-guide/chap-monitoring.adoc defines the attribute, but its OpenDMK console block (unzip opendj-{opendj-version}.zip) has no subs="attributes", and a listing block does not substitute attributes by default.

Change

• server-dev-guide/chap-writing-plugins.adoc (4 links) and reference/appendix-interface-stability.adoc (1 link): point at https://doc.openidentityplatform.org/opendj/apidocs/: index.html for the API as a whole, and the class page itself for PluginType, DirectoryServerPlugin and PasswordValidator. The links are absolute so that they also work in the PDF build.
• install-guide/chap-uninstall.adoc: define :opendj-version: x.y.z, which the pre-processor rewrites like on the other pages.
• admin-guide/chap-monitoring.adoc: subs="attributes" on the OpenDMK console block, as on the other blocks of that page that use the attribute.

Verification

• All four new URLs answer 200. https://doc.openidentityplatform.org/opendj/javadoc/index.html answers 404.
• Rendered the four pages with AsciidoctorJ (asciidoctor 2.0.17) after applying the pre-processor's :opendj-version: rewrite, on master and on this branch:
  • master: {opendj-version} stays literal once in chap-uninstall.html and once in chap-monitoring.html, and 5 hrefs point at ../javadoc/….
  • this branch: no literal {opendj-version} is left (msiexec /x opendj-5.1.1.msi /quiet, unzip opendj-5.1.1.zip), and all 5 hrefs point at …/opendj/apidocs/….
• A scan of every .adoc under opendj-doc-generated-ref/src/main/asciidoc finds no other page that uses {opendj-version} / {opendj-version-short} without defining it, and no other listing block that uses them without subs.

No test is added: the change is documentation only.