Move excel-dna.net website to GitHub pages

So that anyone can send pull-requests and contribute to the website, and also opens the door for devs to keep the site and docs up-to-date as they send PRs to the code repos.

I can easily convert the current website in WordPress to the GitHub Pages / Jekyll / Markdown format and publish into our repo [excel-dna.github.io](https://github.com/Excel-DNA/Excel-DNA.github.io) first and have it running in parallel with the current website.

Once everything is converted, we can make the switch on the domain, to point to GitHub pages, and retire the WordPress site.