Markdown indexing drops text before the first heading, and setext headings #609

#245 was closed asking for a new issue if this still reproduces on the latest release. Two cases still do, on 0.4.4 (md_to_tree) and on main (run_pageindex.py --md_path):

Acme SDK
========

Acme SDK talks to the Acme API. The default timeout is 30 seconds
and every request is retried 3 times.

## Install

pip install acme

## Usage

Call `acme.run()`.

$ python run_pageindex.py --md_path doc.md --if-add-node-summary no --if-add-node-text yes --if-add-doc-description no
"structure": [
  {"title": "Install", "node_id": "0001", "line_num": 7, "text": "## Install\n\npip install acme"},
  {"title": "Usage", "node_id": "0002", "line_num": 11, "text": "## Usage\n\nCall `acme.run()`."}
]

1. Text before the first heading is not in any node, so it is not in the tree. A file with no headings produces an empty structure. The PDF paths keep the same kind of content as a Preface node.
2. Setext headings (Title + === / ---) are not recognized. Here the title is lost along with the intro; in the middle of a file, the section merges into the one before it.

PRs with fixes and tests for both follow.