Add overview.drawio.svg

src/drawio/readme.md

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+# About these files
+
+## `*.drawio`
+
+`*.drawio` contains a diagram created by diagrams.net draw.io, an open-source
+diagramming program. It stores the content of the diagram, which may include
+text, shapes, icons, and images, along with formatting information.[^1]
+
+They are easy to use and their size is affordable.
+
+## `*.drawio.svg`
+
+A `*.drawio.svg` is actually a `svg`(Scalable vector graphics) file but can be
+edited with drawio utils, like extension on VSCode or online
+website(https://www.drawio.com/)
+
+By using `*.drawio.svg` rather than `*.drawio`, we can view the diagram on
+GitHub.
+The SVG file is editable on draw.io and can be embedded on Github.[^2]
+
+[^1]: https://fileinfo.com/extension/drawio
+[^2]: https://github.com/rust-lang/rustc-dev-guide/pull/1725#issuecomment-1627922452