Move directives out to sphinxcontrib-opendataservices

[Bjwebb]

https://github.com/OpenDataServices/sphinxcontrib-opendataservices/

[robredpath]

@stevieflow - this is a bit of maintenance that we've done across our Sphinx sites as part of the OS4D-sponsored sprint to improve our Sphinx stack. Not sure what level of approval it needs - it doesn't actually change anything public-facing, just makes it easier to look after!

[stevieflow]

@Bjwebb if it doesn't change any content, then fine -- @KDuerden for info