docs: name the subject in the configuration heading

"Three containers, three concerns" never said what was in three
containers, so a reader scanning the page could not tell the section
was about configuration. The chime is memorable once you have read it
and invisible before that.

"Configuration lives in three containers" adds the subject, keeps the
compact subject-verb shape of its neighbor "Two layers decide the
roles", and echoes the section's own opening sentence. Deliberately not
converting concepts.rst headings to the task-shaped style used in
customize.rst -- one page explains, the other is looked things up in.

Also fixes the page intro, which still enumerated four ideas and
promised "these four ideas". Adding the two-layer section earlier today
made it five; the enumeration never mentioned how tokens get their
roles.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: derek73

docs/concepts.rst

@@ -3,10 +3,11 @@ How the parser works
 
 ``parse()`` turns a name string into a
 :class:`~nameparser.ParsedName`. This page explains the model behind
-that call: how a string becomes tokens and tokens become fields,
-where configuration lives and why it is split the way it is, why
-parsers are plain values, and what happens when a name is genuinely
-ambiguous. The task pages all build on these four ideas.
+that call: how a string becomes tokens and tokens become fields, how
+those tokens get their roles, where configuration lives and why it is
+split the way it is, why parsers are plain values, and what happens
+when a name is genuinely ambiguous. The task pages all build on these
+five ideas.
 
 From string to name
 --------------------
@@ -71,8 +72,8 @@ you look anything up: if you are teaching the parser a *word*, it goes
 in the :class:`~nameparser.Lexicon`; if you are changing how unclaimed
 words are *arranged*, it goes in the :class:`~nameparser.Policy`.
 
-Three containers, three concerns
---------------------------------
+Configuration lives in three containers
+----------------------------------------
 
 Every piece of nameparser configuration falls into exactly one of
 three places, and which one is decided by a single question: what does