docs(examples): per-example docs pages with mermaid diagrams

[chris-colinsky]

Summary

Second of the docs-phase PRs. Adds a new top-level Examples section to the docs site, sitting between Concepts and Model Providers in the left nav.

• Per-example pages. One page per example (00 through 09) plus an index, each following the same structure: overview / what it teaches / how to run / mermaid graph / reading the output. Cross-links into the relevant concepts/*.md pages in each "what it teaches" section.
• Mermaid wiring. pymdownx.superfences picks up a mermaid custom fence so per-example graph shapes render inline. No external script tag; Material's bundled JS loads the mermaid runtime when it sees a .mermaid element.
• Nav indent. A small left-pad on nested primary-nav links in extra.css so the section hierarchy reads at a glance now that Examples runs 11 entries deep.
• llms.txt coverage. The plugin config picks up the new Examples section plus Model Providers (which had been silently omitted since the plugin was first added), so AI-assistant ingestion covers the full public docs surface.

Out of scope (next PR): README / Quickstart / Concepts re-read against v0.6.0.

Test plan

• uv run --group docs mkdocs build --strict succeeds
• Examples section appears between Concepts and Model Providers in the left nav
• Each per-example page renders its mermaid graph as an SVG (not as a code block)
• Nav children sit slightly to the right of section labels (small indent)
• site/llms.txt contains an ## Examples section listing all 10 example pages plus the index
• site/llms.txt contains a ## Model Providers section listing both pages
• Cross-links from "what it teaches" land on the right concepts pages

[Copilot]

Pull request overview

Adds a new Examples documentation section with per-example walkthrough pages, Mermaid graphs, navigation updates, and llms.txt coverage for the expanded docs surface.

Changes:

• Adds docs pages for examples 00–09 plus an examples index.
• Configures Mermaid rendering and includes Examples/Model Providers in llms.txt sections.
• Adds primary-nav indentation styling for nested nav items.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
mkdocs.yml	Adds Examples nav entries, llms.txt sections, and Mermaid superfences config.
docs/stylesheets/extra.css	Adds indentation for nested primary navigation links.
docs/examples/index.md	Introduces the Examples landing page and catalog.
docs/examples/00-hello-world.md	Documents the hello-world routed pipeline example.
docs/examples/01-routing-and-subgraphs.md	Documents routing and subgraph composition.
docs/examples/02-explicit-subgraph-mapping.md	Documents explicit mapping for reused subgraphs.
docs/examples/03-observer-hooks.md	Documents observer hook usage and output.
docs/examples/04-nested-subgraphs.md	Documents nested subgraph composition.
docs/examples/05-fan-out-with-retry.md	Documents fan-out, retry, timing, and collect-mode behavior.
docs/examples/06-parallel-branches.md	Documents parallel branch execution.
docs/examples/07-multimodal-prompt.md	Documents multimodal prompts and prompt grouping.
docs/examples/08-checkpointing-and-migration.md	Documents checkpoint resume and state migration.
docs/examples/09-tool-use.md	Documents tool-calling agent loop behavior.

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