docs: Add Tor bandwidth files documentation

docs/source/tor_bandwidth_files.rst

+How bandwidth files are shown in the Tor network
+=================================================
+
+Directory authorities' votes
+-----------------------------
+
+moria, using Tor 0.3.5.7:
+
+.. code:: text
+
+    bandwidth-file-headers timestamp=1548181637
+
+https://collector.torproject.org/recent/relay-descriptors/votes/
+
+To appear in Tor v0.4.1.x:
+
+.. code:: text
+
+    bandwidth-file-digest sha256=01234567890123456789abcdefghijkl
+
+https://trac.torproject.org/projects/tor/ticket/26698
+
+Directory authorities' bandwidth file URL
+-----------------------------------------
+
+To appear in Tor v0.4.1.x:
+
+.. code:: text
+
+    /tor/status-vote/next/bandwidth.z
+
+https://trac.torproject.org/projects/tor/ticket/21377