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/feature/api/feature_api.md

@@ -0,0 +1,2 @@
+@dir /feature/api
+@brief feature/api: In-process interface to starting/stopping Tor.