docs(sidebar[tiers]): move argparse to Domain Packages; rename Theme/Fonts/CLI to UX

why: The Furo sidebar is driven by three hidden toctree blocks in
docs/index.md, not by the narrative prose in docs/packages/index.md
or docs/architecture.md.  The previous E6 commit updated the
narrative but not the sidebar, so readers still saw
sphinx-autodoc-argparse listed under "Theme, Fonts & CLI" in every
package page.  Bring the sidebar in line with the architectural
reclassification, and shorten the third-tier caption from
"Theme, Fonts & CLI" to the cleaner "UX" now that argparse has
moved out and only gp-sphinx / sphinx-gp-theme / sphinx-fonts
remain.
what:
- docs/index.md: insert packages/sphinx-autodoc-argparse in the
  "Domain Packages" toctree between api-style and docutils
  (alphabetical order within the tier).
- docs/index.md: rename the third toctree caption from
  "Theme, Fonts & CLI" to "UX"; remove
  packages/sphinx-autodoc-argparse from it.
- Verified by clean-building docs and inspecting the rendered
  sidebar headings (Shared Infrastructure / Domain Packages / UX).

Author: tony

docs/index.md

@@ -112,18 +112,18 @@ packages/sphinx-autodoc-typehints-gp
 :hidden:
 
 packages/sphinx-autodoc-api-style
+packages/sphinx-autodoc-argparse
 packages/sphinx-autodoc-docutils
 packages/sphinx-autodoc-fastmcp
 packages/sphinx-autodoc-pytest-fixtures
 packages/sphinx-autodoc-sphinx
 ```
 
 ```{toctree}
-:caption: Theme, Fonts & CLI
+:caption: UX
 :hidden:
 
 packages/gp-sphinx
 packages/sphinx-gp-theme
 packages/sphinx-fonts
-packages/sphinx-autodoc-argparse
 ```