docs: Add sapphire-localnet appd instructions

[matevz]

This PR:

• adds usage of the new sapphire-localnet that exposes Localnet's rofl-appd

Fixes #2434
Requires oasisprotocol/oasis-web3-gateway#823

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.60%. Comparing base (a241a8a) to head (cd17517).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2442   +/-   ##
=======================================
  Coverage   48.60%   48.60%           
=======================================
  Files         225      225           
  Lines       16128    16128           
=======================================
  Hits         7839     7839           
  Misses       8229     8229           
  Partials       60       60           

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