Add MavlinkDirect docs and more examples

[julianoes]

This adds:

• A guide how to use MavlinkDirect.
• More MavlinkDirect examples.
• A note regarding MavlinkPassthrough deprecation.

[hamishwillee]

FYI, front page en/index.md the "Build and test" isn't rendering. Note that you could also change this from "main" to include v3. I don't do this for PX4 because I use the precise text for some dynamic text config, but we don't in this docset

[hamishwillee]

Why not deprecate now in v3? I have not tested this layout, but consider

Suggested change

	* @warning MavlinkPassthrough will get deprecated in MAVSDK v4 and will only
	* be available built from source.
	* Use MavlinkDirect instead.
	* MavlinkDirect provides enhanced functionality including:
	* @warning MavlinkPassthrough is deprecated.
	* In MAVSDK v4 it will only
	* be available built from source.
	* Use MavlinkDirect instead.
	*
	* MavlinkDirect provides enhanced functionality including:

[julianoes]

I will mark it as deprecated in a few months but I'm allowing for some testing time with MavlinkDirect first. But your suggestion makes sense.

[hamishwillee]

The generated MAVLinkDirect class starts with "Enable direct MAVLink communication using libmav."

Libmav is probably only known to a few people, and this raises questions about dependencies.

I would modify that to talk about what this delivers - e.g. Runtime message parsing with JSON field representation
etc

[hamishwillee]

Also would be cool to show this with Python :-)

[julianoes]

@hamishwillee Also would be cool to show this with Python :-)

mavlink/MAVSDK-Python#801

[julianoes]

FYI, front page en/index.md the "Build and test" isn't rendering. Note that you could also change this from "main" to include v3. I don't do this for PX4 because I use the precise text for some dynamic text config, but we don't in this docset

Yeah, why is that not rendering properly? 😞

Edit: oh, it's essentially an old version of the readme and hasn't been properly updated. Do we really have to have a copy of the readme like this?

Edit 2: I removed the status badges. They are C++ specific anyway.

[hamishwillee]

I think this is good. Added a few last comments.

[julianoes]

Thanks for the help @hamishwillee!

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