docs/contract: Add references to setting up client node for Cipher

[matevz]

Merge after oasisprotocol/docs#215 is implemented.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.92%. Comparing base (e18a28e) to head (3a99b75).
⚠️ Report is 1390 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1150      +/-   ##
==========================================
+ Coverage   67.81%   75.92%   +8.11%     
==========================================
  Files         131      105      -26     
  Lines       11575     9659    -1916     
==========================================
- Hits         7850     7334     -516     
+ Misses       3693     2325    -1368     
+ Partials       32        0      -32     

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