[upstream_utils] Clean up and enable foonathan memory docs (#6990)

Group memory doxygen into one module.

Remove concept alias and add doxygen definitions for foonathan memory.

\concept was added as a doxygen command in 1.9.2 and is meant to be applied to concepts. Inserting them into standard comment paragraphs causes doxygen to interpret the following text as a concept name and add it to the documentation, as well as remove the text from the paragraph.
In the upstream repo, this alias links to markdown documentation, so it's not usable for us anyways.

That, plus adding the doxygen definitions/aliases from upstream cleans up most of the errors/weird output from doxygen for foonathan memory.

docs/theme.css

@@ -2799,4 +2799,4 @@ SOFTWARE.
         left: var(--spacing-medium) !important;
         right: auto;
     }
-}
+}