Onion service DoS guidelines: minor refactor; adds PoW and caching approaches (#312)

This changeset updates the Onion service DoS guidelines page by:

1. Doing a minor refactor, (re)naming and reordering sections.
2. Adding instructions for the PoW defense mechanism.
3. Adding a note about caching strategies.

I tried to do the fewest changes as possible, in order to not increase the number of strings to translate.

Part of #312 (closed).