docs(esplora): add full_scan and sync examples

[kwsantiago]

Adds # Example rustdoc to full_scan and sync on both EsploraExt (blocking) and EsploraAsyncExt (async).

Second in the #2006 follow-up to document the components previously covered by the removed examples/ crates, after #2210 (bitcoind_rpc). Electrum follows next.

Doctests pass under --all-features.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.82%. Comparing base (fcfb7dd) to head (cadf00f).

Files with missing lines	Patch %	Lines
crates/esplora/src/async_ext.rs	0.00%	21 Missing ⚠️
crates/esplora/src/blocking_ext.rs	0.00%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2243      +/-   ##
==========================================
- Coverage   78.32%   77.82%   -0.50%     
==========================================
  Files          30       30              
  Lines        5934     5972      +38     
  Branches      281      281              
==========================================
  Hits         4648     4648              
- Misses       1210     1248      +38     
  Partials       76       76              

Flag	Coverage Δ
rust	77.82% <0.00%> (-0.50%)	⬇️

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