Add conda-forge install instructions

[matthewfeickert]

As tinygp is on conda-forge (https://github.com/conda-forge/tinygp-feedstock) (maybe also worth adding DFM as a maintainer if that's not extra burden) it would be great to also mention this in the install docs (https://github.com/dfm/tinygp/blob/f1addd638d9d6e37f6a1505231a73f207c7a268f/docs/install.md?plain=1), as conda-forge gives super great control over CUDA distributions these days (with help from Pixi).

[dfm]

Sounds good! I'd be very happy to merge such a PR :D