feat: read guides and wiki nav from data

Platform guides, device groups, terminal tabs, pack README manuals and
logo credits move from Python tables to platforms/_guides.yml, checked
against the registry on load. A folder row without a path is the
registry's extract_to, written once.

Wiki pages place themselves in the navigation with order and nav_title
in their front matter. Generated docs/ and mkdocs.yml are unchanged.

wiki/testing-guide.md

@@ -1,5 +1,7 @@
 ---
 level: contribute
+order: 40
+nav_title: Testing
 ---
 
 # Testing guide
@@ -54,7 +56,7 @@ Every module in `tests/`, by the area it guards.
 | Handheld port | stubbed PortMaster dialogs, fake installer | `test_port` |
 | Truth and native export | synthetic + cached originals | `test_export_counts`, `test_export_native_layer`, `test_export_report`, `test_exporters`, `test_truth_data_dir_readers`, `test_truth_diff`, `test_truth_layout` |
 | Scrapers, upstream and provenance | inline sources, synthetic | `test_check_freshness`, `test_dedup`, `test_elec_scrapers`, `test_fbneo_parser`, `test_find_pinned_revision`, `test_hash_merge`, `test_launchers`, `test_launchers_emudeck`, `test_launchers_recalbox`, `test_launchers_retrobat`, `test_launchers_retrodeck`, `test_launchers_retropie`, `test_launchers_rocknix`, `test_mame_devices`, `test_mame_parser`, `test_mame_ref_audit`, `test_profile_sync`, `test_provenance`, `test_refresh_data_dirs`, `test_refresh_failures`, `test_refresh_stale`, `test_retrodeck_scraper_errors`, `test_romset_recipes`, `test_scraper_contract`, `test_upstream`, `test_upstream_index` |
-| Site, docs and repository contracts | rendered pages, sources | `test_contributing_text`, `test_coverage_cache`, `test_data_dir_refs`, `test_dataclass_defaults`, `test_doc_pages`, `test_doc_surfaces`, `test_optional_dependencies`, `test_prose_style`, `test_site_availability`, `test_site_copy`, `test_site_cores`, `test_site_emulator_page`, `test_site_exports`, `test_site_funnel`, `test_site_images`, `test_site_platform_hashes`, `test_site_render`, `test_site_system_links`, `test_site_validation`, `test_site_weight`, `test_site_write`, `test_validate_pr`, `test_validate_pr_flow`, `test_validate_pr_hashes`, `test_workflow_paths` |
+| Site, docs and repository contracts | rendered pages, sources | `test_contributing_text`, `test_coverage_cache`, `test_data_dir_refs`, `test_dataclass_defaults`, `test_doc_pages`, `test_doc_surfaces`, `test_optional_dependencies`, `test_prose_style`, `test_site_availability`, `test_site_copy`, `test_site_content_data`, `test_site_cores`, `test_site_emulator_page`, `test_site_exports`, `test_site_funnel`, `test_site_images`, `test_site_platform_hashes`, `test_site_render`, `test_site_system_links`, `test_site_validation`, `test_site_weight`, `test_site_write`, `test_validate_pr`, `test_validate_pr_flow`, `test_validate_pr_hashes`, `test_workflow_paths` |
 
 ## Test architecture
 
@@ -362,6 +364,34 @@ and reads less on a phone. Facts a newcomer does not act on (legal analysis,
 source lines, annotated runs) are kept, on the page one level down. To see
 the numbers of every page, run `python -m tests.test_site_copy --report docs`.
 
+### Adding a wiki page
+
+A page in `wiki/` is published as soon as it exists, and its front matter
+places it in the navigation. No Python changes.
+
+```yaml
+---
+level: explore           # its tab: start, setup, explore, reference or contribute
+order: 35                # its place in the tab, the smallest first
+nav_title: Save files    # only when the menu names it otherwise than its heading
+---
+```
+
+- `level` picks the tab and the label shown at the top of the page.
+- `order` sorts the tab. Pages are spaced by ten, so a new page fits between
+  two without renumbering. The generated pages (systems, emulators, the gaps
+  and data pages) hold fixed places among them, set in `generate_mkdocs_nav`.
+- `nav_title` is the menu label. Without it the menu shows the page's `# `
+  heading. It is not `title`, which both MkDocs and Astro Starlight read as
+  the page's own title.
+- `nav: false` keeps a page out of the menu on purpose.
+  `getting-started.md` does, since it only links the pages that took its
+  content.
+
+`python scripts/generate_site.py` stops with the page's name when `level` or
+`order` is missing, or when two pages of a tab take the same `order`.
+`tests/test_site_content_data.py` checks the same on every page in `wiki/`.
+
 ## Landing a change
 
 A commit subject is English, 50 characters at most, `type: description` with a