docs: document pixi add channel::package usage example

[darilsobze]

Description

This PR documents how to install a dependency from a specific channel using the channel::package syntax with pixi add.

Pixi already supports package-scoped channels, but this usage was not discoverable from the CLI documentation. The added example shows how to install a single package from a non-default channel without permanently adding that channel to the workspace.

Fixes #{4231}

How Has This Been Tested?

i verified that the example command works as described

TODO

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### Checklist:
<!--- Remove the non relevant items. --->
- [ X] I have made corresponding changes to the documentation

[ruben-arts]

Suggested change

	pixi add https://repo.prefix.dev/bioconda::htslib # (3a)!
	pixi add https://prefix.dev/bioconda::htslib # (3a)!

While repo works it's a deprecated definition.

[ruben-arts]

And the 3a doesn't work it has to be 4, so all the other values have to be moved.

[darilsobze]

so what should i do now? could u please explain more clearly?

[ruben-arts]

Make it item 4 and make 4 -> 5 etc.

[ruben-arts]

You can test this by running pixi run docs it shows you the dev version of the docs website.