Fix incorrect RFC 4648 section references:
- Add RFC URL reference at package level
- Update Error enum documentation to reference correct sections:
- Invalid_Character: Section 3.3 (non-alphabet characters)
- Invalid_Length: Section 6 (base32 block size requirements)
- Malformed_Input: Section 3.2 (padding)
- Fix test file section references to match correct sections
This ensures all RFC references are accurate and adds a link to the
source RFC for reference.