Document reading gzipped text in other encodings

[quinnj]

The reading guide explains gzip and UTF-8 input but does not show how to combine decompression with character conversion. Add a runnable ISO-8859-1 gzip example using CodecZlib and StringEncodings, with both dependencies limited to the docs environment.

The example decompresses first and passes a UTF-8 decoder to CSV.File. It closes the decoder in finally, which also checks for an incomplete final character. It states the source-encoding and in-memory parsing constraints.

Addresses #1091.

Validation: full Documenter build passed, including the example's value assertions; rendered output checked. Independent checks passed for ISO-8859-1, UTF-16LE, UTF-16BE and Windows-1252, plus truncated-character rejection (17 checks). No parser/API changes.

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.75%. Comparing base (c77e39b) to head (cc45b06).

Additional details and impacted files

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##             main    #1209   +/-   ##
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  Coverage   95.75%   95.75%           
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  Files           9        9           
  Lines        5489     5489           
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  Hits         5256     5256           
  Misses        233      233           

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