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, 5 insertions, 0 deletions

diff --git a/src/lib/meminfo/lib_meminfo.md b/src/lib/meminfo/lib_meminfo.md
new file mode 100644
index 0000000000..87f509d648
--- /dev/null
+++ b/src/lib/meminfo/lib_meminfo.md
@@ -0,0 +1,5 @@
+@dir /lib/meminfo
+@brief lib/meminfo: Inspecting malloc() usage.
+
+Only available when malloc() provides mallinfo() or something similar.
+