This adds a top-level section on all the encoding rules for a DiffX

file. This covers how encodings work, how sections inherit or specify

their encodings, and what assumptions a parser can or cannot make based

on the presence of encodings on any given section.
Some other sections that previously discussed encodings now refer to

this new section, except where it's still deemed important to highlight

important information.
One big change is that section header lines are no longer encoded using

the parent section's encoding. Instead, they're encoded using plain

ASCII. This aids in parsing, allowing a parser to skip to any given

section (jumping from section to section based on each section's length

option) without having to keep track of an encoding hierarchy and having

section-specific logic for decoding that given line.

Built the docs. Checked the new Encodings section for typos and bad

links. Checked each section referencing encodings for bad links.