User documentation related to the upcoming Onion Services PoW protection

Create user documentation related to the upcoming Onion Services PoW protection (tpo/core/tor!702 (merged)):

• Sync with Network and Applications Team to coordinate this documentation with C Tor and Tor Browser releases implementing this feature.
• Check the torspec, man page and the onion-pow-example.
• Test the feature. Moved to tpo/onion-services/onion-support#229 (closed).
• Write/update the documentation:
  • Client side: what the users should know, need to do and possible issues (like PoW on low-end mobile devices) (this may go to a different ticket, possibly related to the Tor Browser manual). Being handled at tpo/community/team#93 (closed).
  • Handle tpo/core/torspec!153 (merged) and tpo/core/torspec!155 (merged).
  • Server side: steps to implement the PoW defense by Onion Service Operators at the Onion service DoS guidelines page (!313 (merged)).

References:

• Tor Browser build with PoW support
• onion-pow-example

/cc @gus