redirect gh-pages to readthedocs

[minrk]

for #297

• replace every page with a redirect to readthedocs
• query params, anchor links should be preserved
• adds rel=canonical links to update search engines
• remove unused static assets

Deployed to minrk.github.io/nbgitpuller, so we should be able to test:

• https://minrk.github.io/nbgitpuller/
• https://minrk.github.io/nbgitpuller/link (weird gh-pages special case, but this is what nbgitpuller.link points to)
• https://minrk.github.io/nbgitpuller/link.html
• https://minrk.github.io/nbgitpuller/topic/automatic-merging.html#case-1-the-instructor-changed-a-file-that-the-student-has-not-changed

[consideRatio]

Nice work on this @minrk, I also learned a few things thanks to your clear PR description :)