[#282]:svarga:docs, drop docs/pix/ (unreferenced) + tidy Doxyfile IMAGE_PATH + EXCLUDE

The docs/pix/ tree carried 14 image files (gifs / pngs / svgs / a jpg
or two) from the MkDocs era — financial_eng.jpg, steve-varga.jpg,
perf-{armadillo,stl,struct}.png, sparse-csr.svg, julia-sparse-mat.png,
matrix-base.svg, meta.svg, two other.svg variants, FF-IH_File{Group,Object}.gif,
c-strings-inmemory.png.

Verified unreferenced before removal:
  - 0 markdown `![](pix/...)` image links anywhere in docs/
  - 0 Doxygen `@image` / `\image` directives in source
  - 0 references from kept-input files (docs/index.md, docs/aliases.md,
    docs/curated/, docs/reports/, h5cpp/, examples/, doxy/)
  - 0 images copied into the rendered docs/doxygen/html/ output by
    the previous build

Doxyfile tidied alongside:
  - IMAGE_PATH = ../docs/pix      → removed (target no longer exists)
  - EXCLUDE list trimmed to just  `../docs/doxygen` — the seven mkdocs-era
    EXCLUDE entries (overrides / assets / styles / javascripts / dot /
    icons / pix / examples) all pointed at directories that were deleted
    in the MkDocs retirement commit and are dead config.

Local rebuild via scripts/docs-build.sh is clean (0 warnings); every
key landing page still resolves.

Author: steven-varga