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

1 files changed, 8 insertions, 0 deletions

diff --git a/src/feature/control/feature_control.md b/src/feature/control/feature_control.md
new file mode 100644
index 0000000000..9f1681ea91
--- /dev/null
+++ b/src/feature/control/feature_control.md
@@ -0,0 +1,8 @@
+@dir /feature/control
+@brief feature/control: Controller API.
+
+The Controller API is a text-based protocol that another program (or another
+thread, if you're running Tor in-process) can use to configure and control
+Tor while it is running.  The current protocol is documented in
+[control-spec.txt](https://gitweb.torproject.org/torspec.git/tree/control-spec.txt).
+