Improve GoDoc documentation for brazilcode package

[potatowski]

Description

The GoDoc documentation for the brazilcode Go package is incomplete or outdated. This slows onboarding, limits IDE hover/auto‑complete quality, and increases the risk of incorrect API usage. We need to review and standardize all comments on exported symbols so automatic docs generated via go doc and pkg.go.dev accurately reflect the code.

Goals

1. Inventory all exported symbols

   • List every function, method, type, constant, and variable exported by brazilcode.

2. Review and standardize comments

   • Follow the Go convention: the first sentence starts with the symbol’s name (e.g., // DecodeCPF parses …).
   • Clarify parameters, return values, and possible errors.
   • Add concise runnable examples using Example functions (func ExampleDecodeCPF() { … }).

3. Automate documentation checks

   • Add a CI step running golangci-lint run --enable godot (or similar) to block merges lacking proper comments.

Acceptance Criteria

• 100 % of symbols exported by brazilcode have valid GoDoc comments.
• At least one executable example per public sub‑package passes under go test.
• README includes install instructions and a pkg.go.dev link.
• CI pipeline fails if documentation coverage regresses.
• Review approved by 1 Go Developer and 1 Tech Writer.

Expected Impact

Consistent, accurate docs for brazilcode will boost developer experience, reduce misuse, and shorten ramp‑up time for new contributors.

Helpful Resources

• Official guide: https://go.dev/doc/comment
• Comment linting: golangci-lint rule godot
• Example functions: https://pkg.go.dev/testing#hdr-Examples