Add plotstyle: reusable KIT-branded matplotlib styling toolkit

Introduces a new plotstyle package providing consistent, presentation/thesis-ready
matplotlib figures: a KIT corporate-design color palette (categorical, sequential,
diverging, status), a theme applied via use() (KIT-black bottom spine only, left/bottom
ticks, horizontal gridlines, left-aligned titles, LaTeX text in Latin Modern Sans),
new_figure() with size presets and figure-level title/params subtitles, a
same-size colorbar() helper, style_legend() and panel_label() building blocks, and
savefig(). Ships as an optional "plotting" extra so core gallery installs stay
lightweight, with a full test suite and a runnable example notebook.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pyproject.toml

@@ -50,13 +50,16 @@ dev = [
     "pylint>=2.0",
     "mypy>=0.900",
 ]
+plotting = [
+    "matplotlib>=3.7",
+]
 
 [project.scripts]
 gallery = "gallery.cli:main"
 
 [tool.setuptools]
-packages = ["gallery", "gallery.utils", "gallery.config"]
-package-data = {gallery = ["templates/*", "assets/css/*", "assets/js/*", "config/*"]}
+packages = ["gallery", "gallery.utils", "gallery.config", "plotstyle"]
+package-data = {gallery = ["templates/*", "assets/css/*", "assets/js/*", "config/*"], plotstyle = ["assets/*.mplstyle"]}
 include-package-data = true
 
 [tool.black]