ArduSub: include supported MAVLink messages

[ES-Alexander]

Would be useful if each ArduSub version specifies the MAVLink messages it supports. That should avoid confusing issues where someone tries to send a message in the normal MAVLink spec, but it doesn't exist, or the spec has new parameters that haven't yet been implemented.

Would likely be quite a bit of work to set up initially, but shouldn't be too bad to maintain beyond that, and would be helpful for people wanting to use MAVLink communication to know what is definitely supported.

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Partially completed in #52:

• lists messages and links to firmware source code, but
• we don't yet have verification that the implementations are functional and/or complete
• it'd be nice if the code links were to a specific line/section, rather than just the file that declares/registers support