Fix build errros

[pkra]

See https://travis-ci.org/mathjax/MathJax-website/builds/56442071

• fix feed.xml in head (it's missing the baseurl prefix)
• fix dead links
• fix old articles, see Review content from old site #6
• do something about empty alt-tags (configure htmlproofer or change main page)

[pkra]

For the record, the remaining build errors:

• ./_site/article-about-mathjax-in-e-learning-websites-released/index.html
  • External link http://www.mathjax.org/resources/articles-and-presentations/mathematics-e-learning-community-benefits-from-mathjax/ failed: 404 No error
• ./_site/index.html
  • image img/a11y.svg does not have an alt attribute (line 246)
  • image img/api.svg does not have an alt attribute (line 274)
  • image img/browsers.svg does not have an alt attribute (line 281)
  • image img/code.svg does not have an alt attribute (line 265)
  • image img/gh-mark.svg does not have an alt attribute (line 445)
  • image img/integral.svg does not have an alt attribute (line 232)
  • image img/mailing.svg does not have an alt attribute (line 455)
  • image img/modular.svg does not have an alt attribute (line 239)
  • image img/share.svg does not have an alt attribute (line 464)
• ./_site/mathml-in-html5-problems-with-ie9/index.html
  • External link http://www.mathjax.org/resources/articles-and-presentations/accessible-pages-with-mathjax/ failed: 404 No error
• ./_site/presentations-preview-mathml-support-ideas-on-copy-and-paste/index.html
  • External link http://www.mathjax.org/ext/miner-jmm2010/ failed: 404 No error

[pkra]

ARGH. At first, I thought the remaining errors are timeouts. But now there's a new dead link. Not impressed with htmlproofer 😦; also I'm apparently too stupid to understand its CLI for excluding some domains (like those that have timeout).

I should look into other testers for CI. For now, I'll consider de-activating htmproofer's checks for external URLs; internals are our fault, but externals we maybe shouldn't care that much for.