doc: add splice out example

[Sjors]

The current splicing docs (splice_init, splice_signed and splice_update) have examples for splicing in. I'm still trying to wrap my head around them. But I think it would be useful to add a splice out example. I have a feeling that's easier since you don't (?) need the initial fundpsbt step.

[Sjors]

By the way, I'm assuming the idea is to eventually also have a simple splice command that just works(tm)?

Or maybe seperate ones for splicing in and out, closely mimicking withdraw:

• splice_out channel_id satoshi [destination] [feerate] [minconf]
• splice_in channel_id satoshi [feerate] [minconf] [utxos]

[ddustin]

Adding the splice_out usage example to the splice_init documentation in PR #6677.