[feature]: docs: provide JSON examples for custom channel data

[ZZiigguurraatt]

We have some structs that can be used with json.Unmarshal to decode custom channel data (https://github.com/lightninglabs/taproot-assets/blob/4852be495676014f72f1a8267bdb46fb191695fe/rfqmsg/custom_channel_data.go#L1-L73), however, we could really use some examples of each of these in JSON format for people to easier understand and for people who don't even use golang.

[guggero]

This is going to be refactored by https://github.com/lightninglabs/taproot-assets/pull/1413, see https://github.com/lightninglabs/taproot-assets/blob/0c462c10990ef7a16e1bbf76fd03f65e7d15b567/tapchannelmsg/custom_channel_data_test.go.

[ZZiigguurraatt]

It seems as though lncli now decodes the custom_channel_data (but not custom_records). I'm not sure if this was always the case? Maybe we can just refer people to look at that for an example instead of putting it in the documentation?

[jtobin]

Closing in line with a previous triage effort.