Design: translate from reStructuredText to Markdown (#2198)

For #2196

**What type of PR is this?**

> Documentation

**What package or component does this PR mostly affect?**

> Design

**What does this PR do? Why is it needed?**

Translates Design.rst from reStructuredText to Markdown.

This page is almost entirely obsolete. But let's change the format
before we substantially change the content.

**Which issues(s) does this PR fix?**

Fixes #2196

**Other notes for review**

Author: jayconrod

BUILD.bazel

@@ -74,7 +74,7 @@ filegroup(
         "CODEOWNERS",
         "CONTRIBUTING.md",
         "CONTRIBUTORS",
-        "Design.rst",
+        "Design.md",
         "LICENSE",
         "README.md",
         "WORKSPACE",