README: restructure, add Why/Quick-start/Notes, fix example bugs

Reorganizes the README to lead evaluators in via a "Why u-attributes?" pitch
and a Quick start (with both .with(:initialize) and Micro::Attributes.new
forms), then promotes the feature-overview tables ahead of the deep reference
content. Consolidates the query section into "Reading attributes" and folds
the orphan composition-test paragraph back into the Composition intro. Adds
a Notes section (thread safety, unknown keys in extract_attributes_from,
2.x → 3.x migration pointer).

Also fixes 6 doc-only bugs in the existing examples: missing comma in the
Accept combined example, job_null/job mismatches in the Initialize sections,
"different of a Hash" grammar, a Person.new(1) example misfiled under
#with_attributes, and attribute?(:name) claiming true for a method that
isn't a declared attribute.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: serradura