Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/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