MAINT: Add doc-watch Pixi task

[VeckoTheGecko]

• Closes DOC: Add Sphinx-autobuild to documentation environment #10786
• tested manually

@keewis are you still willing to accept this PR? (and do you still want it to be in a different, separate environment to the normal doc environment?)

[keewis]

yes, this should be fine

and do you still want it to be in a different, separate environment to the normal doc environment?

sphinx-autobuild is small enough, so probably not necessary. I did notice, though, that having a task that hosts the documentation created by the doc task might be useful: sphinx-autobuild sometimes goes into a infinite loop of rebuilding things, which is not helpful (and once we have the jupyterlite PR merged you can't just use file:// open the html files).