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, 15 insertions, 0 deletions

diff --git a/src/lib/sandbox/lib_sandbox.md b/src/lib/sandbox/lib_sandbox.md
new file mode 100644
index 0000000000..dd168c9b13
--- /dev/null
+++ b/src/lib/sandbox/lib_sandbox.md
@@ -0,0 +1,15 @@
+@dir /lib/sandbox
+@brief lib/sandbox: Linux seccomp2-based sandbox.
+
+This module uses Linux's seccomp2 facility via the
+[`libseccomp` library](https://github.com/seccomp/libseccomp), to restrict
+the set of system calls that Tor is allowed to invoke while it is running.
+
+Because there are many libc versions that invoke different system calls, and
+because handling strings is quite complex, this module is more complex and
+less portable than it needs to be.
+
+A better architecture would put the responsibility for invoking tricky system
+calls (like open()) in another, less restricted process, and give that
+process responsibility for enforcing our sandbox rules.
+