Rollup merge of #83420 - GuillaumeGomez:rustdoc-gui-tests-doc, r=CraftSpider

Add documentation for rustdoc-gui tests

I think a bit of documentation doesn't hurt in this case considering how "out of the ordinary" this is.

r? ``@jyn514``

src/test/rustdoc-gui/README.md

+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/