Documentation/blog post: tutorial for downloading IPUMS extract #27

[gavinjqu]

Closes #27

This adds to docs/src/tutorials.md, which is a walkthrough of the IPUMS extract API workflow with IPUMS.jl, modeled on the ipumsr R article.

Overview: supported collections, API key setup, defining a microdata extract in JSON (weights, data quality flags, case selections), extract_submit, extract_download, and reading the result into a DataFrame via parse_ddi + load_ipums_extract. Ends with a full end-to-end script and a Limitations and roadmap section.

Out of scope: the NHGIS / aggregate-data flow. Planning to open a follow-up issue.

Note: a few claims would benefit from a second pair of eyes, specifically the IPUMS sample codes (cps2018_03s / cps2019_03s), the Authorization header format, and the colmetadata key strings used in the reading section. Also, the reading section pulls in a new dependency (CodecZlib) for .dat.gz decompression, and I dropped the outer # Tutorials heading since this is the only article on the page. I'm happy to revise anything if needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.70%. Comparing base (c91f3b5) to head (18b09ec).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #43   +/-   ##
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  Coverage   29.70%   29.70%           
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  Files          27       27           
  Lines         643      643           
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  Hits          191      191           
  Misses        452      452           

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