refactor: show diagrams as themed SVG images

An inline SVG put every label of both layouts, its title and its desc
into the page text, next to the prose that already says the steps: a
text dump of the Steam Deck tab read the steps three times, glued.
Each drawing is now one SVG file per layout and theme under
assets/diagrams, shown through <picture> with alt="". The figure says
its words once, in one screen-reader line and its caption.

scripts/generate_site.py

@@ -95,6 +95,7 @@ import siteentries
 import siteinputs
 import retired_files
 import siteagents
+import sitediagrams
 import sitestarlight
 import sitesteps
 from siteexports import generate_exports_page, load_export_reports
@@ -12497,6 +12498,9 @@ def _generate(args: argparse.Namespace) -> None:
         source = Path("docs_assets") / folder
         if source.is_dir():
             shutil.copytree(source, public / "assets" / dest, dirs_exist_ok=True)
+    # The diagrams and the drawn install steps, one SVG per layout and theme.
+    sitediagrams.write_diagram_files(public / sitediagrams.DIAGRAM_DIR,
+                                     {**sitediagrams.figure_files(), **sitesteps.steps_files()})
     demo_dest = public / "assets" / "demo"
     demo_dest.mkdir(parents=True, exist_ok=True)
     for name in DEMO_ASSETS: