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

2 files changed, 0 insertions, 0 deletions

diff --git a/src/lib/thread/lib_thread.dox b/src/lib/thread/lib_thread.md
index 5870ad790f..5870ad790f 100644
--- a/src/lib/thread/lib_thread.dox
+++ b/src/lib/thread/lib_thread.md
diff --git a/src/lib/thread/threading.dox b/src/lib/thread/threading.md
index a1058c97de..a1058c97de 100644
--- a/src/lib/thread/threading.dox
+++ b/src/lib/thread/threading.md