Move Pin/Unpin Message docs to the Message section

[AlmostSuspense]

trafficstars

I think it makes sense to have pins-related docs in the messages section

[advaith1]

I was unsure about pins and I made a decision to leave them in Channel, but we can move them to Message if people think that's better. (also, should we move Get Pins too?)

would love feedback/votes on this

[Doc94]

Maybe a shortcut the the channel for pins?

[ManHatos]

It would make sense to have either all pin endpoints in messages or leave the GET in channels

[SinisterRectus]

I made a decision to leave them in Channel

What was the reasoning for leaving get pinned messages on the channel page while moving the other get messages endpoints?