Various docs update

- Change the introduction on Routing to make it more self-contained.
- Move the Routing part to a specific page, as it's an advanced topic about platforms.
- Fix the formating of some examples
- Fix the formating of \beginrst blocks in doxygen
- Document the <disk> tag a bit, as well as the filesystem plugin
- Remove the doc about storage from the doxygen doc to be converted

- Document Engine with autodoxy
- Document the C API of Engine and Actor

- Teach autodoxy to produce the doc of C functions (that don't have '::' in their name)
- Teach find_missing to rant about undocumented C functions