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>