[Documentation] Clarify BUILD file names

The documentation currently refers to `BUILD` files with only a small side note that the may be called `BUILD.bazel`, namely where packages are defined.
Instead, add a more prominent note to the page explaining `BUILD` files and clarify there that both namings are valid, that `BUILD.bazel` takes precendence, and that the documentation will talk just about `BUILD` files for simplicity's sake. Additionally, link to that page from the package definition section.

Progress on #4517

Closes #18251.

PiperOrigin-RevId: 529378158
Change-Id: If74903e0a9ffd4410a9c65c1fc9f3e5959fe83a7

site/en/concepts/build-files.md

@@ -10,7 +10,14 @@ build dependency graph abstractly. This section describes the concrete syntax
 used to define a package.
 
 By definition, every package contains a `BUILD` file, which is a short
-program. `BUILD` files are evaluated using an imperative language,
+program.
+
+Note: The `BUILD` file can be named either `BUILD` or `BUILD.bazel`. If both
+files exist, `BUILD.bazel` takes precedence over `BUILD`.
+For simplicity's sake, the documentation refers to these files simply as `BUILD`
+files.
+
+`BUILD` files are evaluated using an imperative language,
 [Starlark](https://github.com/bazelbuild/starlark/){: .external}.
 
 They are interpreted as a sequential list of statements.

site/en/concepts/build-ref.md

@@ -49,11 +49,11 @@ The primary unit of code organization in a repository is the _package_. A
 package is a collection of related files and a specification of how they can be
 used to produce output artifacts.
 
-A package is defined as a directory containing a file named `BUILD` (or
-`BUILD.bazel`). A package includes all files in its directory, plus all
-subdirectories beneath it, except those which themselves contain a `BUILD` file.
-From this definition, no file or directory may be a part of two different
-packages.
+A package is defined as a directory containing a
+[`BUILD` file](/concepts/build-files) named either `BUILD` or `BUILD.bazel`. A
+package includes all files in its directory, plus all subdirectories beneath it,
+except those which themselves contain a `BUILD` file. From this definition, no
+file or directory may be a part of two different packages.
 
 For example, in the following directory tree there are two packages, `my/app`,
 and the subpackage `my/app/tests`. Note that `my/app/data` is not a package, but