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, 6 insertions, 0 deletions

diff --git a/src/lib/trace/lib_trace.md b/src/lib/trace/lib_trace.md
new file mode 100644
index 0000000000..a7a32529b0
--- /dev/null
+++ b/src/lib/trace/lib_trace.md
@@ -0,0 +1,6 @@
+@dir /lib/trace
+@brief lib/trace: Function-tracing functionality API.
+
+This module is used for adding "trace" support (low-granularity function
+logging) to Tor.  Right now it doesn't have many users.
+