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, 7 insertions, 0 deletions

diff --git a/src/feature/dircommon/feature_dircommon.md b/src/feature/dircommon/feature_dircommon.md
new file mode 100644
index 0000000000..359049ecd8
--- /dev/null
+++ b/src/feature/dircommon/feature_dircommon.md
@@ -0,0 +1,7 @@
+@dir /feature/dircommon
+@brief feature/dircommon: Directory client and server shared code
+
+This module has the code that directory clients (anybody who download
+information about relays) and directory servers (anybody who serves such
+information) share in common.
+