Add documentation for rustdoc-gui tests

src/test/rustdoc-gui/README.md

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+The tests present here are used to test the generated HTML from rustdoc. The
+goal is to prevent unsound/unexpected GUI changes.
+
+This is using the [browser-ui-test] framework to do so. It works as follows:
+
+It wraps [puppeteer] to send commands to a web browser in order to navigate and
+test what's being currently displayed in the web page.
+
+You can find more information and its documentation in its [repository][browser-ui-test].
+
+[browser-ui-test]: https://github.com/GuillaumeGomez/browser-UI-test/
+[puppeteer]: https://pptr.dev/