feat: draw site images from data, host them

scripts/sitediagrams.py writes the explainer, install flow, verification modes, MinUI card, pipeline and data layer diagrams as inline SVG from the registry, the platform files, the install manifests and the pipeline step list. A page asks for one with an image comment, which the decoration pass resolves. The two mermaid diagrams of the architecture page are replaced, so no page loads mermaid from unpkg.

Platform logos and system icons are copied once by fetch_site_images.py with their source and owner recorded, and the build rewrites the remote links to the copies. validate_site refuses an image from another host. Three registry logos named a personal avatar instead of the project's (RomM, ROCKNIX, MiSTer FPGA) and MyMinUI's was a GitHub identicon, now dropped.

One 1200x630 sharing card per section, a 404 page with search and the two entry pages, and a list of screenshots that need a real device.

platforms/_registry.yml

@@ -665,7 +665,7 @@ platforms:
     config: romm.yml
     status: active
     extract_to: library/bios/
-    logo: https://avatars.githubusercontent.com/u/168586850
+    logo: https://avatars.githubusercontent.com/u/164058018
     scraper: romm
     source_url: https://raw.githubusercontent.com/rommapp/romm/master/backend/models/fixtures/known_bios_files.json
     source_format: json
@@ -727,7 +727,7 @@ platforms:
     config: rocknix.yml
     status: active
     extract_to: /storage/roms/bios/
-    logo: https://avatars.githubusercontent.com/u/152618023
+    logo: https://avatars.githubusercontent.com/u/163176886
     scraper: rocknix
     schedule: weekly
     target_scraper: rocknix_targets
@@ -897,7 +897,7 @@ platforms:
     config: misterfpga.yml
     status: active
     extract_to: /media/fat/games/
-    logo: https://avatars.githubusercontent.com/u/47264183
+    logo: https://avatars.githubusercontent.com/u/29345272
     scraper: misterfpga
     source_url: https://raw.githubusercontent.com/ajgowans/BiosDB_MiSTer/db/bios_db.json.zip
     source_format: mister_downloader_db
@@ -980,7 +980,6 @@ platforms:
     # minarch hands each core SDCARD_PATH/Bios/<TAG>
     # (workspace/all/minarch/minarch.c:3532).
     extract_to: <SD card>/
-    logo: https://avatars.githubusercontent.com/u/10894566
     scraper: myminui
     # minarch hands each core a folder and names no file (workspace/all/minarch/minarch.c:3532):
     # there is no BIOS list of its own to export corrected.