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/feature.md b/src/feature/feature.md
new file mode 100644
index 0000000000..acc3487e55
--- /dev/null
+++ b/src/feature/feature.md
@@ -0,0 +1,7 @@
+@dir /feature
+@brief feature: domain-specific modules
+
+The "feature" directory has modules that Tor uses only for a particular
+role or service, such as maintaining/using an onion service, operating as a
+relay or a client, or being a directory authority.
+