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FLASHING.md says how to take a newer version of the colleague's tree (copy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/docs/api.md

@@ -0,0 +1,10 @@
+Title: API Documentation
+
+
+An API documentation can be generated directly from source code with [Doxygen](http://www.doxygen.nl).
+If Doxygen is installed on your system, you may simply call `doxygen` in the repository's root directory.
+The generated HTML documentation can then be found at *doxygen/html/index.html*.
+
+We also maintain an [online copy](https://www.vanetza.org/doxygen/index.html) which is updated every now and then.
+Of course, your locally generated documentation will be more up-to-date in most cases.
+Please consider to generate the documentation locally if you need it to match a particular or more recent version of Vanetza.