[docs] Add documentation scrape test.

Fairly simple script that scrapes code blocks out of the documentation and runs
them through the DSL interpreter main binary -- already found a bunch of places
there were inconsistencies!

Should generally help make sure our docs are more up to date and sound.

For future work, for DSLX "snippets" (things that are not complete code blocks
that are displayed, but that are used to show DSL constructs), we may want to
define a "hidden" prelude before the snippet that makes the block executable,
and have some way to delimit what subset of the snippet is shown when rendered.

Fixes #378

PiperOrigin-RevId: 375183932

docs_src/BUILD

@@ -0,0 +1,25 @@
+# Copyright 2021 The XLS Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Targets for documentation files.
+
+package(
+    default_visibility = ["//xls:xls_internal"],
+    licenses = ["notice"],  # Apache 2.0
+)
+
+filegroup(
+    name = "dslx_reference",
+    srcs = ["dslx_reference.md"],
+)