[DOC] Host a clickable dune2 example report suite

[eleanorfrajka]

Summary

Publishes a live, self-contained example report suite alongside the docs so readers can explore the real HTML output interactively instead of only reading a description. Fulfils the "worked example report is planned for a future release" note that previously sat at the bottom of reports.rst.

Changes

docs/source/_static/demo/dune2/ (new): the five real dune2_1_2026 report pages — Summary, Stack, Grid, and two per-instrument pages (microcat 2941, aquadopp 9920) — copied from the test fixtures, preserving the instrument/ subdirectory so the header cross-links resolve. Each page is self-contained (all figures base64-embedded; the only external reference is the GitHub link in the footer). Sphinx copies _static/ verbatim, so they publish at .../oceanarray/_static/demo/dune2/….

docs/source/reports.rst: adds a "Live example" section near the top — a click-through thumbnail (opens the Summary page) plus direct links to all five pages — and removes the now-obsolete placeholder note.

docs/Makefile: adds a demo-report target that refreshes the hosted suite from the fixture reports (which the test suite regenerates), so the example can be kept current with one command.

.gitattributes: marks docs/source/_static/demo/** as binary (= -diff -merge -text) on top of the existing linguist-generated, so git treats the demo reports as opaque blobs — no textual diff, no merge, no line-ending normalization — while still tracking them for Sphinx to serve. Golden fixtures keep linguist-generated only (they stay diffable, since their masked images make diffs meaningful on rebaseline).

Notes

The committed snapshot (~2.6 MB across five pages) will drift from the current output when report layout or figures change; make demo-report regenerates it. A future option is to generate it during the docs CI build instead of committing it, but that is deferred.

Breaking changes

None — documentation and build tooling only.