docs(sidebar[tiers]): regroup into Domain/UX/Utils/Internal

why: "Shared Infrastructure" bundled three heterogenous packages
under one caption — sphinx-ux-badges and sphinx-ux-autodoc-layout
are visual/styling primitives that naturally belong alongside
sphinx-fonts, while sphinx-autodoc-typehints-gp is a type-rendering
utility that doesn't fit either.  The single generic caption made
it hard for a reader browsing the sidebar to reason about what each
package is for at a glance.  Split into four purpose-named
captions so the navigation matches the conceptual categories.
what:
- docs/index.md: replace the three hidden toctrees (Shared
  Infrastructure / Domain Packages / UX) with four captioned
  toctrees:
    * Domain Packages — the six autodoc-* extensions (unchanged).
    * UX — sphinx-fonts, sphinx-ux-autodoc-layout, sphinx-ux-badges
      (the three visual/styling primitives).
    * Utils — sphinx-autodoc-typehints-gp (type-rendering utility
      that replaces sphinx-autodoc-typehints + sphinx.ext.napoleon).
    * Internal — gp-sphinx (metapackage / coordinator) and
      sphinx-gp-theme (furo sub-theme), the two packages readers
      don't install on their own.
- Alphabetise entries within each toctree for predictability.
- No narrative (docs/architecture.md, docs/packages/index.md)
  changes in this commit; the architecture tiers describe
  dependency layers and remain three-tier, while the sidebar
  captions describe navigation buckets.

Author: tony

docs/index.md

@@ -98,15 +98,6 @@ project/index
 history
 ```
 
-```{toctree}
-:caption: Shared Infrastructure
-:hidden:
-
-packages/sphinx-ux-badges
-packages/sphinx-ux-autodoc-layout
-packages/sphinx-autodoc-typehints-gp
-```
-
 ```{toctree}
 :caption: Domain Packages
 :hidden:
@@ -123,7 +114,22 @@ packages/sphinx-autodoc-sphinx
 :caption: UX
 :hidden:
 
+packages/sphinx-fonts
+packages/sphinx-ux-autodoc-layout
+packages/sphinx-ux-badges
+```
+
+```{toctree}
+:caption: Utils
+:hidden:
+
+packages/sphinx-autodoc-typehints-gp
+```
+
+```{toctree}
+:caption: Internal
+:hidden:
+
 packages/gp-sphinx
 packages/sphinx-gp-theme
-packages/sphinx-fonts
 ```