Show how to use via `uv` without `pipx`

[Julian]

The README already shows how to use build with uv, but does so assuming someone's also using pipx.

Given that uv has functionality which overlaps with pipx, is it perhaps worth changing this section to assume someone is using uv's own functionality for installing build? (Doing so is not as simple as replacing pipx with uvx in the command, as build reiles on entry-points to tell pipx to invoke it via -m).

Specifically, is it worth changing that section to say:

uv run --with 'build[uv]' -m build --installer=uv

?

[layday]

I don't think the pipx invocation should be changed to uv's, but examples can be provided for both. If you're running build via uv, you don't need the uv extra, and the uvx invocation can be simplified to:

uvx --from build pyproject-build --installer uv