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 184.108.40.206, all README files become just "Go to the source code of this file" links.
The README in the core's root directory is the exception; it's actually wikitext. gerrit 27117 creates a symlink README.mediawiki because [[ https://github.com/github/markup#markups | github supports the .mediawiki extension ]], unlike doxygen.