DOC: Add Sphinx-autobuild to documentation environment

What is your issue?

I realised while doing #10732 that the contributing instructions for building documentation is the following:

https://github.com/pydata/xarray/blob/a3bd20d85ff03c70e54f3c777d1b2f385f514074/doc/contribute/contributing.rst#L452-L479

I find this build then manually open to be a bit out of my normal workflow (and for what I've seen quite a bit), as opposed to using sphinx-autobuild something like

sphinx-autobuild docs docs/_build

Which launches a server and rebuilds the documentation on change to the documentation.

Would this be something devs are interested in?

Changes needed:

• add sphinx-autobuild as a dependency
• add a task pixi run doc-watch to spin up the server (and still have pixi run doc in case people just want to build and not watch)
• update contributing instructions

I can do this alongside the PR for #10732 (preferred from my end) or in a subsequent PR if you'd prefer that.

I searched the issue tracker - couldn't find a mention of sphinx-autobuild

Start with doc/contribute/contributing.rst and the existing pixi run doc workflow, then locate the project configuration for documentation dependencies and tasks. Done means a pixi run doc-watch command starts the documented server and rebuilds on documentation changes, while the contributing instructions explain both workflows.