docs/howto/add_a_uavcan_message.md is outdated

[antoinealb]

Hello @antoinealb, our 🤖 robots detected that docs/howto/add_a_uavcan_message.md has not been reviewed in the last 6 month, and could be outdated.

How do I fix this ?

Please review docs/howto/add_a_uavcan_message.md, do any required changes, change the freshness header to add yourself as a recent reviewer, then commit it to this repository.
If the file is not required anymore, you can simply do a commit to delete it, it will not be taken into account anymore.