docs: Add bandwidth authorities documentation

docs/source/bandwidth_authorities.rst

+Bandwidth authorities in metrics
+=================================
+
+Current bandwidth authorities
+-----------------------------
+
+.. image:: images/bwauth.*
+   :alt: bandwidth authorities in metrics
+
+https://metrics.torproject.org/rs.html
+
+(flag:Authority)
+
+Bandwidth Authorities - Measured Relays past 7 days
+---------------------------------------------------
+
+.. image:: images/bwauth_measured_7days.png
+   :alt: bandwidth measured in the past 7 days
+
+https://consensus-health.torproject.org/graphs.html
+
+Bandwidth Authorities - Measured Relays past 90 days
+----------------------------------------------------
+
+.. image:: images/bwauth_measured_90days.png
+   :alt: bandwidth measured in the past 90 days
+
+https://consensus-health.torproject.org/graphs.html