zcash_client_sqlite: Start documenting the table structures

[str4d]

The existing pins for the current database structure are moved out of the verify_schema test, so they can serve the dual purpose of documentating the database structure.

The documentation needs significantly more detail than this PR currently includes; this is intended as a starting point from which we can continually improve our documentation going forward.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.03%. Comparing base (87e2308) to head (cf1922c). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1429 +/- ## ========================================== + Coverage 63.01% 63.03% +0.02% ========================================== Files 126 127 +1 Lines 14772 14784 +12 ========================================== + Hits 9308 9319 +11 - Misses 5464 5465 +1 ```

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