docs: warn that endpoints outside the filter paths are unfiltered

[alukach]

Adds a warning to the ITEMS_FILTER_PATH docs: requests to paths matched by neither ITEMS_FILTER_PATH nor COLLECTIONS_FILTER_PATH are proxied unfiltered. Upstreams that expose aggregation (/aggregate, /collections/{id}/aggregate), collection search (e.g. /collections-search) or queryables endpoints need to extend these patterns or block those endpoints.

Docs only.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.76%. Comparing base (be008ba) to head (176fbed).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #215   +/-   ##
=======================================
  Coverage   89.76%   89.76%           
=======================================
  Files          30       30           
  Lines        1348     1348           
  Branches      180      180           
=======================================
  Hits         1210     1210           
  Misses         97       97           
  Partials       41       41           

Flag	Coverage Δ
unittests	89.76% <ø> (ø)

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