Update toolchains documentation to use JSR-330 terminology and links

[slawekjaranowski]

• Replaced Plexus-specific references with JSR-330 annotations (@Named, @Singleton).
• Updated outdated links to relevant JSR-330 component documentation and plugin source tree.

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Pull request overview

This PR updates the Maven Toolchains Plugin site documentation for custom toolchains to use current JSR-330 terminology and to point to a more appropriate project “source tree” link.

Changes:

• Updated the “plugin source tree” site link to scm.html.
• Replaced Plexus component-annotation guidance with JSR-330 guidance (@Named, @Singleton) for ToolchainFactory.

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