Document the Nix32 base-32 variant used for store path digests and
hash output. The new page covers:
- The 32-character alphabet (omitting e, o, u, t)
- Byte order differences from base-16 encoding
Also update references throughout the manual to link to the new page.
1. Fix confusing wording that might imply unnecessary double-hashing.
2. Add references to specifics of base-32 encoding.
3. Fix incorrect description that sha256 hash of `fingerprint` is
truncated. "Truncated" is actual wording used in Nix theses, but it has
unusual meaning, that is better conveyed by word "compressed", which is
used by the reference C++ implementation.
4. Clarify details of base16 encoding.
