Document finished features

[CathalMullan]

Both internal and public API needs documentation.

cargo watch --notify --shell "cargo doc --document-private-items"

EDIT:
Doctest support is missing from nextest/llvm-cov, which is annoying for coverage metrics.

EDIT 2:
Suprised there isn't a GHA to upload docs to pages, and comment a link on the PR.

Guess this PR is going to devolve into GHA shenanigans.

EDIT 3:
Pages doesn't support sub-paths.
Need to wait until the 'preview' feature releases: https://github.com/actions/deploy-pages

We can add a docs preview to the main branch, which will still be useful for pre-release checks.

EDIT 4:
Noticed some inconsistencies around our error messages.

EDIT 5:
Leaving the main, crate level documentation to a later date.

[codspeed-hq]

CodSpeed Performance Report

Merging #126 will not alter performance

_{Comparing 125-begin-documenting-crate (c08ccf3) with main (3a35573)}

Summary

✅ 16 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files	Patch %	Lines
src/path.rs	60.00%	2 Missing ⚠️

Files	Coverage Δ
src/constraints.rs	0.00% <ø> (ø)
src/decode.rs	78.12% <100.00%> (-18.75%)	⬇️
src/errors/constraint.rs	100.00% <ø> (ø)
src/errors/decode.rs	0.00% <ø> (-100.00%)	⬇️
src/errors/delete.rs	73.33% <ø> (ø)
src/errors/insert.rs	76.47% <ø> (ø)
src/errors/route.rs	100.00% <100.00%> (ø)
src/node/delete.rs	77.77% <ø> (ø)
src/node/insert.rs	91.63% <100.00%> (+0.06%)	⬆️
src/node/search.rs	98.02% <100.00%> (+0.48%)	⬆️
... and 3 more