Tidy up the "Benchmarks" section of the documentation

[noonio]

This area needs some love: https://hydra.family/head-protocol/unstable/benchmarks

A lot of the links 404; there's some outdated information on computing script sizes; and the actual metrics we care about are a little hard to track down.

We should:

• The wording probably needs to be revised
• Remove or correct dead links
• Restructure it around important metrics and categories
• Provide (nix) commands to run benchmarks locally and obtain csv files
• Have a graph (or more) that we compute, by hand, for released versions
• The tx-costs benchmarks seem odd; for example for fanout we only see 10 parties - https://hydra.family/head-protocol/benchmarks/transaction-cost#fanout-transaction-costs