For example https://doc.wikimedia.org/mediawiki-core/master/php/README.html , it has garbled "Variables" and "Variables Documentation" sections documenting a details variable, etc. Likewise other README files like maintenance/README. Other doxygen pages even link to these bogus variable definitions, e.g. "details" here. This is by design according to the doxygen FAQ:
[prior to 1.8.8] doxygen parsed all files with an unknown extension as C files which could lead to undesired results
includes/filebackend/README adapts to this behavior by starting with /*! and including Doxygen directives, but changing every README to appear as C seems like overkill.
Instead, Extension mapping says we can specify the mapping for .no_extension. Most README files with no extension are just plaintext, but insanely, doxygen doesn't support a text file format (it's even processing .txt files as C, e.g. APACHE-LICENSE-2.0.txt). So perhaps EXTENSION_MAPPING = ".no_extension=md" (markdown) is the best.
Note the behavior will change when and CI updates to a newer Doxygen version. In my local tests with doxygen 188.8.131.52, all README files become just "Go to the source code of this file" links.