Fix broken docs build and add professional branding/features

The docs.yml CI workflow was completely broken: mkdocs.yml's nav and
doc/index.md both referenced doc/prd.md and doc/task.md, which were
untracked from git in an earlier commit. `mkdocs build --strict` on a
fresh checkout (verified via a real clone simulating CI) aborted with
5 warnings -- the site has never successfully deployed. Removed those
nav entries and the dead links; fixed comparison.md's cross-reference
to point at judge-voting-benchmark.md instead of the no-longer-public
prd.md.

Also added real professional polish, verified by serving the site
locally and screenshotting the result:
- doc/assets/ghost-icon.svg used as theme.logo/favicon (reuses the
  hero animation's ghost path).
- navigation.tabs/sections/footer, search highlight/share, deep
  purple accent palette instead of material's default blue.
- pymdownx.details, pymdownx.tabbed, pymdownx.tasklist, footnotes,
  code annotations -- markdown features the existing guide content
  can actually use.

Author: parthmax2

doc/assets/ghost-icon.svg

@@ -85,7 +85,7 @@ Legend: ✅ confirmed via docs/README · ⚠️ marketed but not confirmed in do
   with G-Eval-style metrics is materially the same pitch, more mature, with
   50+ pre-built metrics vs. ghostrun's five.
 
-## What's still a genuinely open question (see [prd.md](prd.md) and the literature review in this conversation)
+## What's still a genuinely open question (see the [judge-voting benchmark](judge-voting-benchmark.md))
 
 Whether **caching a single judge verdict** (ghostrun's and DeepEval's shared
 default) is a safe engineering tradeoff given the now-published judge flip-rate

doc/comparison.md

@@ -38,8 +38,6 @@ pytest test_ghostrun_example.py
 
 - [Judge-voting benchmark](judge-voting-benchmark.md)
 - [Comparison with other tools](comparison.md)
-- [Product spec](prd.md)
-- [Task tracker](task.md)
 
 Source, issues, and the changelog live on
 [GitHub](https://github.com/parthmax2/ghostrun).

doc/index.md

@@ -13,32 +13,56 @@ site_dir: site
 
 theme:
   name: material
+  logo: assets/ghost-icon.svg
+  favicon: assets/ghost-icon.svg
+  icon:
+    repo: fontawesome/brands/github
   palette:
     - media: "(prefers-color-scheme: light)"
       scheme: default
+      primary: deep purple
+      accent: deep purple
       toggle:
         icon: material/brightness-7
         name: Switch to dark mode
     - media: "(prefers-color-scheme: dark)"
       scheme: slate
+      primary: black
+      accent: deep purple
       toggle:
         icon: material/brightness-4
         name: Switch to light mode
   features:
     - navigation.instant
     - navigation.tracking
     - navigation.top
+    - navigation.tabs
+    - navigation.sections
+    - navigation.footer
     - search.suggest
+    - search.highlight
+    - search.share
     - content.code.copy
+    - content.code.annotate
+    - content.tabs.link
     - content.action.edit
+    - toc.follow
 
 markdown_extensions:
   - meta
   - toc:
       permalink: true
   - admonition
-  - pymdownx.highlight
+  - footnotes
+  - pymdownx.details
+  - pymdownx.tasklist:
+      custom_checkbox: true
+  - pymdownx.highlight:
+      anchor_linenums: true
+  - pymdownx.inlinehilite
   - pymdownx.superfences
+  - pymdownx.tabbed:
+      alternate_style: true
 
 nav:
   - Home: index.md
@@ -52,6 +76,3 @@ nav:
   - Research:
       - Judge-voting benchmark: judge-voting-benchmark.md
       - Comparison with other tools: comparison.md
-      - Product spec: prd.md
-  - Project:
-      - Task tracker: task.md