Add executable documentation examples and dedicated CI

[vgvoleg]

Add executable topic reference examples with named snippet regions and a separate Examples (ydb_tech) job that discovers scenario folders. Preserve the original documentation variants and keep fixture preparation, verification and cleanup outside the excerpts.

Keep both synchronous and asynchronous variants, including manual sequence numbers, transaction query results and both auto-partitioning modes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.67%. Comparing base (6a849d5) to head (0409ca9).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #915   +/-   ##
=======================================
  Coverage   82.67%   82.67%           
=======================================
  Files          99       99           
  Lines       12961    12961           
  Branches     1269     1269           
=======================================
  Hits        10716    10716           
+ Misses       1794     1793    -1     
- Partials      451      452    +1     

Flag	Coverage Δ
integration	80.44% <ø> (ø)
unit	49.21% <ø> (ø)

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see 1 file with indirect coverage changes

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