docs: Add bandwidth authorities documentation

Bandwidth authorities in metrics

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Current bandwidth authorities

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.. image:: images/bwauth.*

   :alt: bandwidth authorities in metrics

https://metrics.torproject.org/rs.html

(flag:Authority)

Bandwidth Authorities - Measured Relays past 7 days

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.. 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

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.. image:: images/bwauth_measured_90days.png

   :alt: bandwidth measured in the past 90 days

https://consensus-health.torproject.org/graphs.html