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/compress/lib_compress.md b/src/lib/compress/lib_compress.md
new file mode 100644
index 0000000000..c43f223fe7
--- /dev/null
+++ b/src/lib/compress/lib_compress.md
@@ -0,0 +1,6 @@
+@dir /lib/compress
+@brief lib/compress: Wraps several compression libraries
+
+Currently supported are zlib (mandatory), zstd (optional), and lzma
+(optional).
+