Chore: add docs and tests for decimal vectors

[connortsui20]

Tracking Issue: #5028

Note that I'm probably going to move where this is placed in the next PR but the content will be the same.

[codecov]

Codecov Report

❌ Patch coverage is 96.38009% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.06%. Comparing base (8bd2cbb) to head (8f50b05).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-vector/src/decimal/generic_mut.rs	96.38%	8 Missing ⚠️

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[a10y]

usually i try and # out the import blocks in doc tests

[connortsui20]

For the longer examples like these I find that I am usually copy and pasting it somewhere else, and in these cases I actually get really annoyed when imports are hidden. For smaller examples though I agree