docs(demo): add badge demo hub and sphinx_pytest_fixtures showcase page

why: Badge rendering varies across themes, zoom levels, and light/dark
modes. A dedicated demo page provides a visual reference for all badge
permutations to verify rendering correctness during development.
what:
- Add docs/demo/index.md as demo hub with description and toctree
- Add docs/demo/sphinx-pytest-fixtures-badges.md with sections for
  every badge permutation: plain, scope (session/module/class), kind
  (factory/override_hook), state (autouse/deprecated), and
  combinations (session+factory, session+autouse)

Author: tony

docs/demo/index.md

@@ -0,0 +1,24 @@
+---
+myst:
+  html_meta:
+    "description": "Visual demo and test pages for libtmux documentation components"
+---
+
+(demo-index)=
+
+# Demo
+
+Visual showcases and test pages for documentation components. Useful for
+verifying badge rendering, theme compatibility, and extension behaviour.
+
+## Sphinx extensions
+
+- [sphinx\_pytest\_fixtures badges](sphinx-pytest-fixtures-badges.md) —
+  All badge permutations (scope, kind, autouse, deprecated) in both the
+  fixture index table and individual fixture cards.
+
+```{toctree}
+:hidden:
+
+sphinx-pytest-fixtures-badges
+```

docs/demo/sphinx-pytest-fixtures-badges.md

@@ -0,0 +1,108 @@
+# sphinx\_pytest\_fixtures — Badge Demo
+
+Visual reference for all badge permutations. Use this page to verify badge
+rendering across themes, zoom levels, and light/dark modes.
+
+Each section documents one badge combination. The index table at the top
+gives a compact overview of all permutations at once.
+
+```{py:module} spf_demo_fixtures
+```
+
+## Fixture Summary
+
+```{autofixture-index} spf_demo_fixtures
+```
+
+---
+
+## Plain (FIXTURE badge only)
+
+Function scope, resource kind, not autouse. Shows only the green FIXTURE badge.
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_plain
+```
+
+---
+
+## Scope badges
+
+### Session scope
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_session
+```
+
+### Module scope
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_module
+```
+
+### Class scope
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_class
+```
+
+---
+
+## Kind badges
+
+### Factory kind
+
+Return type `type[str]` is auto-detected as factory — no explicit `:kind:` needed.
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_factory
+```
+
+### Override hook
+
+Requires explicit `:kind: override_hook` since it cannot be inferred from type.
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_override_hook
+   :kind: override_hook
+```
+
+---
+
+## State badges
+
+### Autouse
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_autouse
+```
+
+### Deprecated
+
+The `deprecated` badge is set via the `:deprecated:` RST option on `py:fixture`.
+`autofixture` does not support `:deprecated:`; use `py:fixture` instead.
+
+```{eval-rst}
+.. py:fixture:: demo_deprecated
+   :deprecated: 1.0
+   :replacement: demo_plain
+   :return-type: str
+
+   Return a deprecated value. Use :fixture:`demo_plain` instead.
+```
+
+---
+
+## Combinations
+
+### Session + Factory
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_session_factory
+```
+
+### Session + Autouse
+
+```{eval-rst}
+.. autofixture:: spf_demo_fixtures.demo_session_autouse
+```