doc: add documentation for IsUniq and IsUniqBy

[headless-samber]

Description

Adds documentation for IsUniq and IsUniqBy functions introduced in #870.

Changes

• docs/data/core-isuniq.md: Doc page for IsUniq
• docs/data/core-isuniqby.md: Doc page for IsUniqBy
• README.md: Added TOC entries and sections for both functions
• docs/static/llms.txt: Added both helpers to the list
• Cross-references: Updated similarHelpers in core-uniq.md, core-uniqby.md, core-uniqbyerr.md, core-uniqmap.md

Notes

• Playground URLs are intentionally left empty since IsUniq/IsUniqBy are not yet in a tagged release.

Supersedes #894

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.60%. Comparing base (cafe8ec) to head (4844c04).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #896   +/-   ##
=======================================
  Coverage   92.60%   92.60%           
=======================================
  Files          32       32           
  Lines        5082     5082           
=======================================
  Hits         4706     4706           
  Misses        273      273           
  Partials      103      103           

Flag	Coverage Δ
unittests	92.60% <ø> (ø)

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