Add a navigable and audited Backtest User Guide

[stefan-jansen]

Why

The 0.1.9 documentation release has a User Guide tab but no overview page or clear path from the Book Guide. The homepage also contains a run_backtest example with undefined names. The Results guide's HTML tearsheet example uses an undefined value and omits its visualization dependency. This PR is the first corrective pass for #120.

Changes

• Add /user-guide/ with task and asset-setting routes to the existing topic pages and complete tutorials; link it from the homepage and Book Guide.
• Replace the broken homepage snippet with a complete, checked run_backtest example.
• Replace the Results tearsheet snippet with a complete example using ml4t-diagnostic[viz]==0.1.4; run it against the installed wheel in CI and release verification.
• State prerequisites for contextual snippets on each topic page and remove unsupported absolute claims about framework matching, vectorization, and test counts.

Verification

• uv run mkdocs build --strict passed. The rendered guide link checker resolved 2,005 content links, including page and section anchors, and checked 29 external destinations. It now runs in PR CI and the docs workflow.
• The Book Guide checker resolved 22 companion paths at its pinned revision.
• Twelve documentation contract tests and pre-commit run --all-files passed.
• A fresh Python 3.12 environment installed the built 0.1.10.dev3+gb2d2702df wheel and ran every default documentation example. After installing ml4t-diagnostic[viz]==0.1.4, the diagnostic handoff and full HTML tearsheet examples passed.
• A static audit parsed every User Guide Python block and found no missing public Broker, DataFeed, Result, or OrderType member names and no unknown BacktestConfig keywords. Contextual fragments remain labeled with their prerequisites; this audit does not establish every prose claim.

Issue #120 remains open until the reviewed guide is merged, deployed, and checked at the canonical route. This PR should not be read as a claim of 100% documentation correctness.

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