Fix markdown heading levels

[kevinbader]

Problem description

c4builder currently assumes that markdown files don't use h1 (#) and h2 (##) headers. For example:

If you don't know this, you might be tempted to start with a h1 heading, which is what you'd usually do. Then it looks like this:

This is probably not what you want. Even more obvious in case of generated markdown:

# test

* [Overview](#Overview)
* [src](#src)
  * [foo](#foo)

---

## Overview

# Top-level readme h1

top-level readme content

And here's the PDF view:

Thoughts

• Most projects use a h1 heading with their project name in the top-level readme file. If that is present, there's no need to render the project name separately or define a HOMEPAGE_NAME. (Related to #9.)
• To prevent forcing users to use only level 3+ headings, c4builder could alter heading levels in the build process.

Thoughts?