feat: Full support of min level via sections

[thompson-tomo]

Closes: #140
Progresses: #78

This has the toc headers grouped into sections with the sections formed based on min level. In the case where multiple sections are found it exits, with stderr.

By grouping the headers into sections we can easily support sections in v3 and it provides an out of the box solution for handling the edge case of having multiple headers below the min level.

It is easier to review with whitespace off aka https://github.com/thlorenz/doctoc/pull/368/changes?w=1

[Copilot]

Pull request overview

Adds “min header level via sections” support to DocToc by grouping headers into sections derived from --minlevel, enabling subsection-only TOCs and introducing a guardrail when multiple sections are detected.

Changes:

• Implement section-building logic in lib/transform.js and update marker/header processing to support minlevel-driven sections.
• Refactor HTML header extraction in lib/get-html-headers.js to return richer node/range info for downstream section logic.
• Add/adjust tests and fixtures for minlevel behavior and the “multiple sections” warning case; update README docs.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lib/transform.js	Core implementation of section detection/grouping and minlevel-driven TOC generation behavior.
lib/get-html-headers.js	Refactors HTML header parsing to support the updated transform/section logic.
doctoc.js	Removes the --minlevel > 2 restriction so higher min levels are accepted.
test/transform.js	Adds a minlevel-focused transform test case.
test/transform-stdout.js	Adds a CLI test asserting the multi-section warning output.
test/fixtures/readme-with-sections.md	New fixture containing multiple sections (and front matter) for the CLI test.
README.md	Documents section-based behavior for --minlevel and the multi-section failure mode.

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