Use prettier to format the documentation (#5708)

To enforce the 100 character line limit in markdown files introduced in
https://github.com/astral-sh/uv/pull/5635, and to automate the
formatting of markdown files, i've added prettier and formatted our
markdown files with it.

I've excluded the changelog and the generated references documentation
from this for having too many changes, but we can also include them.

I'm not particular on which style we use. My main motivations are
(major) not having to reflow markdown files myself anymore and (minor)
consistence between all markdown files. I've chosen prettier for similar
reason as we chose black, it's a single good style that's automated and
shared in the community. I do prefer prettier's style of not breaking
inside of a link name though.

This PR is in two parts, the first adds prettier to CI and documents
using it, while the second actually formats the docs. When merge
conflicts arise, we can drop the last commit and regenerate it with `npx
prettier --prose-wrap always --write BENCHMARKS.md CONTRIBUTING.md
README.md STYLE.md docs/*.md docs/concepts/**/*.md docs/guides/**/*.md
docs/pip/**/*.md`.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

scripts/packages/README.md

@@ -1,3 +1,4 @@
 # packages
 
-A collection of packages used to test editable installs and bespoke behaviors in packaging setups and definitions.
+A collection of packages used to test editable installs and bespoke behaviors in packaging setups
+and definitions.