Conversion to Markdown

This converts the documentation to Markdown, fixing #3.

For the sake of posting a handy PDF, I used [GitHub's release feature](https://github.com/rgbkrk/joern-docs/releases). If you like that, it's certainly one means of posting the PDF outside of git itself.

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There are a couple tricky bits that won't be maintained across this LaTeX -> Markdown conversion.

* Table of Contents
* Bibtex entry

The table of contents could be generated, especially for use with GitHub pages. The BibTex entry I'm unsure what to do with. I ended up linking directly to your paper on mlsec.org.

I have not removed the LaTeX sources, but they likely should so as not to have duplicate doc sources to update.