Keep vanetza-idf in obu-firmware, so a plain clone builds the firmware

obu-firmware builds against the vanetza-idf C-ITS library, which until now
came from the colleague's microbu-esp32c5 tree beside the repository and was
not tracked here, so a clone of this repository could not build the firmware
it ships. The library alone is now part of obu-firmware, as
obu-firmware/external/vanetza-idf: their external/vanetza-idf at commit
cf4b99f, unchanged (9775 files; see its PROVENANCE.md). CMake takes it from
there by default; -DVANETZA_IDF_DIR still points the build elsewhere.

The rest of the colleague's tree (their own VAM firmware, PKI tooling,
station-link Python tools, the V2X2MAP bridge) stays out of this repository
and gitignored; nothing is pushed to their repository. NOTES.md, docs/06,
TODO.md and the pcap verifier's usage line point at the new location.

obu-firmware/external/vanetza-idf/mkdocs.yml

@@ -0,0 +1,22 @@
+site_name: Vanetza - Your open-source ETSI C-ITS protocol stack
+site_dir: www
+repo_url: https://github.com/riebl/vanetza
+theme:
+    name: readthedocs
+    custom_dir: mkdocs/
+    highlightjs: false
+    collapse_navigation: false
+extra_css:
+    - css/extra.css
+    - css/tango.css
+extra_javascript:
+    - js/extra.js
+markdown_extensions:
+    - admonition
+    - smarty
+    - sane_lists
+    - codehilite:
+        guess_lang: false
+plugins:
+    - awesome-nav
+    - search