Condense the 2.1 release notes and give them an intro

The section ran 4908 words over 26 entries, averaging 188 with a
longest of 635. That is a specification, not release notes, and the
substance was a second copy: usage.rst already carries the honorific
peel, the comma and 间隔号 rules, the reach limits, the period rule and
the vocabulary rationale. Nobody finds any of it in a changelog.

Entries are now a summary and a link, 2137 words in the same 26
entries. Where content had no docs home it moved rather than being
dropped: decomposed input gets a usage.rst section of its own, being a
real concern for anyone reading names off macOS.

Adds an intro paragraph in the shape 2.0.0 uses, saying what the
release is for and how to turn it off.

Em-dashes went from 75 to 0. At one every 65 words they had stopped
marking anything.

The NFD example in the new docs section is corrected from the one the
release log implied: for an UNSPLIT decomposed name family holds the
whole string, so comparing it to a one-syllable literal fails for the
wrong reason. The spaced form splits normally and shows the encoding
point properly, and both lines are verified.

Author: derek73