Convert wiki pages to a /docs directory of Markdown

Converted all wiki links to Markdown, adding H1s and updating internal links as necessary. Temporarily renamed the "Home" page to a "README" page, so the GitHub web UI provides an obvious start page.

Each internal link currently points to a `*.md` file, not a `*.html` file, so that the link works in the GitHub web interface. Once these files are generated as HTML and hosted somewhere, the internal links should be changed to point to *.html files.