Redesign website

[corollari]

Hi! I saw (here and there) that you are currently planning a redesign for the page, so I decided to try to help with it. This PR is a full overhaul of the page.

You can see a live version of the web on unlicense.dev or https://corollari.github.io/unlicense.org/.

Changes:

• Add The Unlicense as the License of the repo (/LICENSE file)
• Stop using markdoc (it's abandoned and, according to this, it doesn't work anymore).
• Use pandoc, battle tested software that will continue to be maintained for years to come
• Remove all social media (see Remove tracking buttons #67 for justification)
• Apply Explain usage in package registries such as npm or PyPI #65
• Simplify system (now all the content is just in README.md)
• Add CONTRIBUTING.md to explain the development process (I know that usually CONTRIBUTING.md is used for the Code of Conduct and stuff like that, but there doesn't seem to be a need for that in this repository. If the use of that filename is not appropriate we could change it to HACKING.md to follow the standard[1])
• Changed the link to the list of devs that unlicense their works from http://twitter.com/theunlicense/developers to https://twitter.com/theunlicense/lists/developers/members as the current link redirects to another url that is a list of retweets (not very useful).
• Removed link to the google mailing list (there's been only a single new post since 2016 and it hasn't received any answers).
• Added testing and automatic building with travis (therefore README.md and the web won't fall out of sync and we can make sure that any
• Removed dead links
• Changed almost all (the ones that wouldn't break) the links to https

Rationale for the redesign

README.md serves both for the github repo and the website.

Tests added

• Checks build with pandoc
• Checks dead links
• Checks misspellings

Deployment

Travis automatically builds and deploys the web to github pages

Regarding licensing of the code

• pandoc.css is licensed under CC0
• README.md was already under the public domain

Other ideas:

• Make the authors list a wiki page so that anyone can edit it to add themselves. It could also be removed as it's never going to be complete and would be require high maintenance to support properly.
• Should a link to the github repo be added on the web? That would help people propose changes or fix things.

Other stuff:

• I've also registered the domain unlicense.dev and I was thinking of simply redirecting it to unlicense.org. If you'd prefer to have control over it I can transfer it to you.
• I'm willing to become the maintainer of this repo if you want.

[1] https://en.wikipedia.org/wiki/README#As_a_generic_term