Merged various dataset "posts" into a single page

Very WIP.

* Breaks some URLs (probably wants
https://github.com/nhshackday/nhshackday.github.io/pull/105 in for
redirects)

* Still suffers from "tools is not a good name"
(https://github.com/nhshackday/nhshackday.github.io/issues/94)

* Does not have auto-linked URLs (as per GitHub Flavoured Markdown)

BUT

* Does provide a way to make it clear about what's open and what's not
(https://github.com/nhshackday/nhshackday.github.io/issues/108)

* Does fix the "is this an API that's a source of data or a dataset"
question

* Does mean that rather than the slightly obscure "link" format (which
has caught people out:
https://github.com/nhshackday/nhshackday.github.io/pull/110) is replaced
with "write Markdown, like everywhere else"