Reorganize docs nav into three top-level tabs (#92)

The sidebar had grown long enough to scroll at full height because
navigation.sections rendered every section fully expanded. Switch to
navigation.tabs (with navigation.expand) so the top-level areas
become header tabs and each tab's sidebar shows its pages open.

Regroup the nav under three tabs. Documentation holds the home page,
Getting Started, Concepts, Model Providers, and Reference; Examples
and Patterns are their own tabs. No pages or URLs change.

Author: chris-colinsky

mkdocs.yml

@@ -34,7 +34,8 @@ theme:
     - content.code.annotate
     - content.tabs.link
     - navigation.indexes
-    - navigation.sections
+    - navigation.tabs
+    - navigation.expand
     - navigation.tracking
     - navigation.footer
     - search.highlight
@@ -100,27 +101,33 @@ markdown_extensions:
       custom_checkbox: true
 
 nav:
-  - OpenArmature: index.md
-  - Getting Started:
-    - Quickstart: getting-started/index.md
-  - Concepts:
-    - concepts/index.md
-    - State and reducers: concepts/state-and-reducers.md
-    - Graphs: concepts/graphs.md
-    - Composition: concepts/composition.md
-    - Middleware: concepts/middleware.md
-    - Fan-out: concepts/fan-out.md
-    - Parallel branches: concepts/parallel-branches.md
-    - LLMs: concepts/llms.md
-    - Prompts: concepts/prompts.md
-    - Observability: concepts/observability.md
-    - Checkpointing: concepts/checkpointing.md
-  - Patterns:
-    - patterns/index.md
-    - Parameterized entry point: patterns/parameterized-entry-point.md
-    - Tool dispatch as node: patterns/tool-dispatch-as-node.md
-    - Session as checkpoint resume: patterns/session-as-checkpoint-resume.md
-    - Bypass if output exists: patterns/bypass-if-output-exists.md
+  - Documentation:
+    - OpenArmature: index.md
+    - Getting Started:
+      - Quickstart: getting-started/index.md
+    - Concepts:
+      - concepts/index.md
+      - State and reducers: concepts/state-and-reducers.md
+      - Graphs: concepts/graphs.md
+      - Composition: concepts/composition.md
+      - Middleware: concepts/middleware.md
+      - Fan-out: concepts/fan-out.md
+      - Parallel branches: concepts/parallel-branches.md
+      - LLMs: concepts/llms.md
+      - Prompts: concepts/prompts.md
+      - Observability: concepts/observability.md
+      - Checkpointing: concepts/checkpointing.md
+    - Model Providers:
+      - model-providers/index.md
+      - Self-hosted vLLM: model-providers/vllm.md
+      - Authoring a Provider: model-providers/authoring.md
+    - Reference:
+      - reference/index.md
+      - openarmature.graph: reference/graph.md
+      - openarmature.llm: reference/llm.md
+      - openarmature.prompts: reference/prompts.md
+      - openarmature.checkpoint: reference/checkpoint.md
+      - openarmature.observability: reference/observability.md
   - Examples:
     - examples/index.md
     - Hello, world: examples/00-hello-world.md
@@ -134,17 +141,12 @@ nav:
     - Checkpointing and migration: examples/08-checkpointing-and-migration.md
     - Tool use: examples/09-tool-use.md
     - Langfuse observability: examples/10-langfuse-observability.md
-  - Model Providers:
-    - model-providers/index.md
-    - Self-hosted vLLM: model-providers/vllm.md
-    - Authoring a Provider: model-providers/authoring.md
-  - Reference:
-    - reference/index.md
-    - openarmature.graph: reference/graph.md
-    - openarmature.llm: reference/llm.md
-    - openarmature.prompts: reference/prompts.md
-    - openarmature.checkpoint: reference/checkpoint.md
-    - openarmature.observability: reference/observability.md
+  - Patterns:
+    - patterns/index.md
+    - Parameterized entry point: patterns/parameterized-entry-point.md
+    - Tool dispatch as node: patterns/tool-dispatch-as-node.md
+    - Session as checkpoint resume: patterns/session-as-checkpoint-resume.md
+    - Bypass if output exists: patterns/bypass-if-output-exists.md
 
 extra:
   # Hide the "Made with Material for MkDocs" footer.