docs: streamline README getting started

[zeitlinger]

What changed

• streamlined README.md around a clearer Getting Started flow
• moved detailed adaptive-run behavior out of the README and into docs/cli.md
• simplified top-level docs navigation and trimmed low-value reference duplication
• clarified agent guidance for mise run lint:fix
• fixed CLI docs so --allow-fixed and --time are documented, including the FLINT_ALLOW_FIXED env var binding

Why

The README had grown less smooth to follow, especially around setup and where to find deeper details. This change keeps the README focused on onboarding and moves CLI-specific behavior into the CLI docs.

Impact

• new users get a clearer install/use path
• example setup now points to a realistic external repo
• CLI-specific behavior is easier to find in docs/cli.md

Validation

• mise run lint:fix

[Copilot]

Pull request overview

This PR streamlines onboarding documentation by refocusing README.md on a clearer Getting Started flow and moving deeper operational/reference detail into dedicated docs, while updating the Rust registry/doc-generation logic to support the new doc structure (overview tables, upstream links, and generated CLI flag tables).

Changes:

• Reworked README.md Getting Started and top-level docs navigation; moved “adaptive runs” details into docs/cli.md.
• Added registry metadata (project_url, config_doc_url, overview placements) and updated doc generation/tests to render overview tables + linked linter/config references.
• Added an auto-generated flint run flags table in docs/cli.md with sync tests.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/runner.rs	Updates test helper Check struct literal to include new registry fields.
src/registry/types.rs	Adds overview-related types and new Check metadata fields + builder methods.
src/registry/checks.rs	Annotates built-in checks with upstream URLs and overview placements.
src/registry/tests.rs	Reworks README/linters docs sync test to validate new overview tables and linked details.
src/registry/mod.rs	Re-exports new overview types (with a new unused-import allowance).
src/main.rs	Adds cli_docs_tests module and introduces env var binding for --allow-fixed.
src/linters/license_header.rs	Adds native config display hint for generated docs tables.
src/cli_docs_tests.rs	New tests to generate/validate docs/cli.md run-flags table and enforce env var bindings.
README.md	Streamlines Getting Started, simplifies nav, and replaces registry table with categorized overview tables.
docs/linters.md	Renames/reframes linter reference and adds generated overview section + linked headings/configs.
docs/cli.md	Replaces hand-written flags list with generated flags+env-var table; moves adaptive-runs content here.
default.json	Removes a package rule entry no longer needed.
.github/renovate.json5	Removes README-related custom managers and a package rule entry to match preset changes.
.github/renovate-tracked-deps.json	Removes README tracking now that README snippets/version pins were removed.
src/readme_snippets.rs	Deleted (README snippet markers/keys no longer used).
src/bin/sync-readme-snippets.rs	Deleted (snippet syncing replaced by tests/generation approach).

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