Explain build frontend vs. build backend (#11094)

We regularly get questions why `uv build` is missing certain files or
using the wrong build tag, when that's done by the build backend and
part of the build backend's docs. I tried to clarify this difference and
to redirect users to look at the tool's docs instead of wondering why
uv's docs don't explain that.

---------

Co-authored-by: Ed Morley <501702+edmorley@users.noreply.github.com>
Co-authored-by: Zanie Blue <contact@zanie.dev>

docs/concepts/projects/build.md

@@ -8,6 +8,14 @@ distributions (wheels). The former is typically a `.tar.gz` or `.zip` file conta
 source code along with some additional metadata, while the latter is a `.whl` file containing
 pre-built artifacts that can be installed directly.
 
+!!! important
+
+    When using `uv build`, uv acts as a [build frontend](https://peps.python.org/pep-0517/#terminology-and-goals)
+    and only determines the Python version to use and invokes the build backend. The details of
+    the builds, such as the included files and the distribution filenames, are determined by the build
+    backend, as defined in [`[build-system]`](./config.md#build-systems). Information about build
+    configuration can be found in the respective tool's documentation.
+
 ## Using `uv build`
 
 `uv build` can be used to build both source distributions and binary distributions for your project.