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Migrate the SAXS data-analysis content into the RTD/Sphinx docs as user/saxs/data_analysis.md, and turn the empty saxs.md stub into a landing page linking to it. Source: cSAXS documentation wiki, page "SAXS-data-analysis" https://gitea.psi.ch/cSAXS/documentation/wiki/SAXS-data-analysis (git@gitea.psi.ch:cSAXS/documentation.wiki.git, as of 408e535) Converted Gitea-flavored Markdown to MyST: added cross-reference labels, bash code fences, and {note} admonitions. Also bundle the SAXS standards plate datasheet (SAXS_standards_plate.pdf), previously at cSAXS/documentation/attachments/, linked via the MyST {download} role so Sphinx copies it into the build. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>