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, 0 insertions, 0 deletions

diff --git a/src/feature/api/feature_api.dox b/src/feature/api/feature_api.md
index 3065c000aa..3065c000aa 100644
--- a/src/feature/api/feature_api.dox
+++ b/src/feature/api/feature_api.md