Sparql link 404

[mdlincoln]

Addresses #289

1. Replaces a broken link to a SPARQL tutorial in the "Further Reading" section with a link to the W3C overview of SPARQL (which is actually a better place for more in-depth study, anyway!)
2. Adds a note to the "Linking multiple SPARQL endpoints" section explaining that the Europeana endpoint is down as of 2016-09-12 until further notice (pending new info from the folks at Europeana Labs)

@wcaleb I bolded the advisory note, but if there's some other class baked into the PH CSS that would be better to use for this deprecation note, please let me know / edit the PR

[wcaleb]

There is a deprecated: true flag that produces this message at the top, but I don't know if it would be appropriate in this case. I think on some other recent lessons @acrymble has written a more descriptive message like the one that you have in this PR. Thoughts?

[mdlincoln]

Agreed, I would not use the deprecated flag because it is inappropriate. If you're fine with just a boldface message at the start of the affected section, then we're all set. I suppose you could make it red or something, too?