build: include bazel build file in aio playground code (#51485)

Include the bazel build file in the aio playground code so it contains the same bazel
targets as the regular examples.

PR Close #51485

.bazelignore

@@ -6,7 +6,6 @@
 node_modules
 dist
 aio/node_modules
-aio/content/example-playground
 aio/tools/examples/shared/node_modules
 aio/tools/examples/shared/example-scaffold
 

aio/content/examples/examples.bzl

@@ -131,7 +131,6 @@ def docs_example(name, test = True, test_tags = [], test_exec_properties = {}, f
     native.filegroup(
         name = "files",
         srcs = native.glob(["**"], exclude = [
-            "BUILD.bazel",
             "**/node_modules/**",  # Node modules may exist from the legacy setup.
         ]),
     )