docs(whats-new): add argparse-domain advancement; bump counts

why: The What's New page counted seven advancements and the
"Shared layout stack" section listed five domain packages.  The
argparse domain that landed in the recent seven-commit series (a
real Sphinx Domain subclass with :argparse:* roles and two
auto-generated indices) is a peer advancement that the page did
not yet describe.  Update the counts and introduce the new section
so readers arriving via the sidebar's "What's new" card see the
whole picture.
what:
- Opening line: "seven major advancements" -> "eight".
- "Shared layout stack" section: "The five domain packages" -> "The
  six domain packages"; add argparse to the {doc} list,
  alphabetically between api-style and docutils.
- Insert a new "## argparse Sphinx domain" section between
  "Shared layout stack" and "Three-tier package organization".
  Describes the Domain subclass, the four :argparse:* xref roles,
  the two auto-generated indices (argparse-programsindex,
  argparse-optionsindex), and the intersphinx compatibility
  contract with :option: / std:cmdoption.

Author: tony

docs/whats-new.md

@@ -3,7 +3,7 @@
 # What's new
 
 The `autodoc-improvements` branch introduces a unified **autodoc design
-system** — seven major advancements that transform how the documentation
+system** — eight major advancements that transform how the documentation
 stack works.  See the {doc}`gallery` for a visual showcase.
 
 ## New packages
@@ -27,8 +27,9 @@ light/dark theming.
 
 ## Shared layout stack
 
-The five domain packages
+The six domain packages
 ({doc}`api-style <packages/sphinx-autodoc-api-style>`,
+{doc}`argparse <packages/sphinx-autodoc-argparse>`,
 {doc}`docutils <packages/sphinx-autodoc-docutils>`,
 {doc}`fastmcp <packages/sphinx-autodoc-fastmcp>`,
 {doc}`pytest-fixtures <packages/sphinx-autodoc-pytest-fixtures>`,
@@ -37,6 +38,18 @@ now all share the same layout, badge, and typehint infrastructure.  A
 change in the foundational layout package propagates instantly and
 consistently.
 
+## argparse Sphinx domain
+
+{doc}`sphinx-autodoc-argparse <packages/sphinx-autodoc-argparse>` now
+ships a real Sphinx `Domain` subclass.  Programs, options, subcommands,
+and positional arguments are individually addressable via
+`:argparse:program:`, `:argparse:option:`, `:argparse:subcommand:`, and
+`:argparse:positional:` xref roles.  Two auto-generated indices —
+`argparse-programsindex` (alphabetised programs) and
+`argparse-optionsindex` (options grouped by program) — give a workspace
+overview.  `:option:` / `std:cmdoption` continues to resolve for
+intersphinx consumers.
+
 ## Three-tier package organization
 
 The workspace has been restructured into a clear {doc}`three-tier