Nest pipeline docs and add example configs page

[lg345]

What changed

Restructures the nav so everything about the YAML pipeline lives under one
section, and adds a page showing the full config for each analysis mode.

New "YAML pipeline" nav section, with children:

• Guide (YAML_PIPELINE_GUIDE.md)
• Example configs (new)
• Step reference (Overview + four category pages)
• Source (Model / Controller / Analysis steps)

The old top-level "Step reference" and "Source (YAML pipeline)" sections are
folded in here. The notebook getting-started pages move to a "Getting started"
section (renamed from "Examples" to avoid clashing with the new configs page).

docs/example_configs.md embeds the six analysis-mode YAML files live via
pymdownx snippets, so the docs track the real files in examples/:
static XES, time-resolved XES, 2D XAS, temporal XAS, droplet XES, RIXS.

Test

mkdocs build with check_paths: true on snippets passes, so every include
resolves; spot-checked the built page to confirm the YAML renders.

[Copilot]

Pull request overview

This PR restructures the MkDocs navigation to group all YAML pipeline documentation under a single “YAML pipeline” section, and adds a new documentation page that embeds complete example pipeline YAML configurations directly from the repo’s examples/ directory.

Changes:

• Reorganized mkdocs.yml nav to nest guide, example configs, step reference, and source pages under a new “YAML pipeline” section, and renamed “Examples” to “Getting started”.
• Added docs/example_configs.md, embedding six end-to-end pipeline YAML files via pymdownx.snippets.
• Updated pymdownx.snippets configuration to set base_path and enable check_paths so snippet includes are validated during mkdocs build.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
mkdocs.yml	Restructures the docs nav and configures snippet includes (with path checking) to support live-embedded example YAML files.
docs/example_configs.md	New page that documents and embeds full pipeline configs for each supported analysis mode from examples/.

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