Download / install instructions web page

[jar600]

Addresses #36.

I'm not saying this PR is ready for merge; my purpose here is to signal that it is wanting review.

Commit 014f73a is on the web here.

Remaining tasks before deployment

• Decide whether the download is a .tar.zip file or a .tgz file (this is for @vkmvkmvkmvkm )
• If .tgz then update the documentation
• Trim the set of notebooks included and bump the version number to 0.2 (@riastradh in progress)
• Automate the application of the css and boilerplate, perhaps using 'bundle' like the main web site?? I don't yet understand how this would be done given that it's not the main web site.

[riastradh-probcomp]

test

[riastradh-probcomp]

test test

[riastradh-probcomp]

one more test