Add Figure Panel Builder plugin page

[Jay2owe]

Adds the ImageJ plugin page for Figure Panel Builder.

The page documents:

• update-site installation and the five-stage guided workflow
• multi-series LIF handling and supported input limitations
• representative-section guidance and descriptive section-level group comparison
• the zoomable layout, external-label, annotation, calibration, and orientation editors
• assembled figures, full-resolution panel assets, project-wide exports, and reproducibility records
• Quick Grid, macro/headless use, Java API, dependencies, version, and license

Verified against the v0.1.0 source, tests, public release, and live update site. Front matter parses successfully, public URLs return HTTP 200, and the PR contains only _pages/plugins/figure-panel-builder.md.

Related update-site registry PR: imagej/list-of-update-sites#215