Doxygen: rename all .dox files to end with .md

Using a standard ending here will let other tools that expect
markdown understand our output here.

This commit was automatically generated with:

   for fn in $(find src -name '*.dox'); do \
      git mv "$fn" "${fn%.dox}.md"; \
   done

src/lib/string/lib_string.md

@@ -0,0 +1,13 @@
+@dir /lib/string
+@brief lib/string: Low-level string manipulation.
+
+We have a number of compatibility functions here: some are for handling
+functionality that is not implemented (or not implemented the same) on every
+platform; some are for providing locale-independent versions of libc
+functions that would otherwise be defined differently for different users.
+
+Other functions here are for common string-manipulation operations that we do
+in the rest of the codebase.
+
+Any string function high-level enough to need logging belongs in a
+higher-level module.