Move concept guides from component sites to maven.apache.org #1751

Several how-to pages that explain Maven concepts live in component sites, so they publish only when that component is released, and some duplicate a guide that already exists here. This moves them into maven.apache.org guides, where they publish with the next site build, and leaves the component sites with their goal reference and component-specific examples.

Each topic is one maven-site PR and one component PR:

1. The maven-site PR adds or extends the guide and adds a Redirect Permanent line in content/filtered-resources/.htaccess for every published URL it replaces. The model XSDs link to the SCM and toolchains pages, so those redirects are required.
2. The component PR deletes the pages and links to the guide. It merges after the site is published.

The guides stay plain Markdown, with the current plugin release written in each snippet, so they render the same on GitHub and need no Velocity escaping.

Overlaps with existing guides:

• Move the archiver manifest and reproducible build examples into the guides #1752, Link to the manifest guide on maven.apache.org maven-archiver#437 Manifests: maven-archiver examples/manifest*, classpath into guide-manifest and guide-archive-configuration
• Move the archiver manifest and reproducible build examples into the guides #1752, Link to the reproducible builds guide on maven.apache.org maven-artifact-plugin#280 Reproducible builds: maven-artifact-plugin reproducible into guide-reproducible-builds
• Merge the site plugin's content guides into the site guides #1761, Move the site creation and Mermaid pages to maven.apache.org maven-site-plugin#1340 Creating a site: maven-site-plugin creating-content, multimodule, creatingskins, i18n, filtering-content into guide-site and skins/
• Add one Mermaid diagrams page for Doxia #1760, Move the Mermaid diagrams page to maven.apache.org maven-doxia-sitetools#726 (and Move the site creation and Mermaid pages to maven.apache.org maven-site-plugin#1340) Mermaid diagrams: one copy under doxia/ instead of the near-identical pages in maven-site-plugin and maven-doxia-sitetools
• Merge the toolchains plugin pages into the toolchains guide #1757, Move the toolchains pages to maven.apache.org maven-toolchains-plugin#213, Move the Java target, toolchain and multi-release pages to maven.apache.org maven-compiler-plugin#1193 Toolchains: maven-toolchains-plugin toolchains/index, jdk, custom and maven-compiler-plugin compile-using-different-jdk into guide-using-toolchains
• Replace the Wagon HTTP guide with the current configuration reference #1755, Add a guide to deploying over HTTP, FTP and SSH #1762, Link to the HTTP configuration guide on maven.apache.org maven-wagon#1021, Move the HTTP, FTP and SSH deployment examples to maven.apache.org maven-deploy-plugin#737 HTTP and deployment: maven-wagon http-configuration into guide-http-settings; maven-deploy-plugin deploy-ftp, deploy-http, deploy-ssh-external, deploy-network-issues into the transport and deployment guides

Concepts without a guide yet:

• Add an SCM URLs page to the POM reference #1754, Link to the SCM URLs page on maven.apache.org maven-scm#1427 SCM URLs: maven-scm scm-url-format, scms-overview and the per-SCM URL sections into the POM reference
• Add guides to targeting a Java version and building multi-release JARs #1756, Move the Java target, toolchain and multi-release pages to maven.apache.org maven-compiler-plugin#1193 Java targets: maven-compiler-plugin set-compiler-release, set-compiler-source-and-target, multirelease
• Add a guide to filtering resources #1758, Link to the filtering resources guide on maven.apache.org maven-resources-plugin#561 Filtering and encoding: maven-resources-plugin filter, escape-filtering, encoding
• Add a guide to migrating a site from APT to Markdown #1759, Move the APT to Markdown migration guide to maven.apache.org maven-doxia-converter#175 APT to Markdown: maven-doxia-converter migrating-apt-to-markdown into doxia/
• Move the dependency resolution misconceptions page from Resolver #1753, Move the common misconceptions page to maven.apache.org maven-resolver#2250 Resolver concepts: only common-misconceptions moves. remote-repository-filtering, local-repository, expected-checksums, transporter-known-issues and transitive-dependency-resolution stay in Resolver, because each documents configuration or behaviour of one Resolver line, and the 1.9.x and 2.x sites publish separately.
• Compare and move only where they overlap: maven-plugin-tools using-annotations, the maven-invoker-plugin examples and maven-indexer how-it-works were compared with their guides; none overlaps enough to move.

Not in scope: generated reference (goal pages, enforcer rules, lifecycle and default-binding pages, the super POM), and component-specific examples.