[DOCS] Add docs to hash and hash to docs

[kevinzwang]

Also fixes link in Expression.approx_count_distinct

[codspeed-hq]

CodSpeed Performance Report

Merging #2821 will improve performances by ×4.7

_{Comparing kevin/hash-docs (60da539) with main (7bee225)}

Summary

⚡ 2 improvements
✅ 14 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	kevin/hash-docs	Change
⚡	test_count[1 Small File]	24 ms	10.3 ms	×2.3
⚡	test_show[100 Small Files]	227.2 ms	48.9 ms	×4.7

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.32%. Comparing base (7bee225) to head (212352c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2821   +/-   ##
=======================================
  Coverage   63.32%   63.32%           
=======================================
  Files        1008     1008           
  Lines      114165   114165           
=======================================
+ Hits        72292    72293    +1     
+ Misses      41873    41872    -1     

Files with missing lines	Coverage Δ
daft/expressions/expressions.py	93.50% <ø> (ø)

... and 1 file with indirect coverage changes

[github-actions]

🚀 Deployed on https://deploy-preview-2821--daft-docs-preview.netlify.app