Pin/unpin dock autohide

### Problem

Sometimes the dock needs to be hidden

Sometimes temporarily the dock needs to stay visible for a bit

It can be a bit annoying to hunt for the setting in the settings panel when the dock is right here

### Proposal

This is purely me seeing a feature in another dock and going "I want it!!!!" but the dock from MainstreamOS has a "Pin" togglebutton. When toggled, it keeps the dock "always visible". When untoggled the dock uses the intelligent hiding setting.

On a squonched dock in the center everything is crowded, but if [Appmenu in dock](https://github.com/elementary/dock/issues/337) and [Expand the dock](https://github.com/elementary/dock/issues/597) together gets in the dock suddenly has a ton of empty space that makes more items "ok"

If the proposal is rejected, we might just implement it as a patch for our install. It feels very much out of scope

### Prior Art (Optional)

_No response_

Start by reviewing how the dock's existing intelligent hiding setting is exposed and how the dock currently switches between hidden and visible states. Compare the proposal with issues #337 and #597, then define the pin toggle's behavior and UI placement. Done means a pin control keeps the dock always visible when enabled and restores intelligent hiding when disabled.

Written by the indexing model from the issue text.