docs: publish “RustBoard technical design” doc (#4350)

Summary:
This design doc describes at a high level the architecture of RustBoard,
a component of TensorBoard designed to provide fast loading of log
directories.

wchargin-branch: docs-design-rustboard

docs/design/images/.gitattributes

@@ -0,0 +1,17 @@
+# Copyright 2020 The TensorFlow Authors. All Rights Reserved.
+#
+# 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.
+# ==============================================================================
+
+# SVG generated by Google Docs and manually cropped in Inkscape. Unwieldy.
+rustboard_data_flow.svg -merge -diff