Update or remove the change management whitepaper

[pp-mo]

I thought we'd removed this so it is no longer in the built docs.
But it turns out it is still "there", though not linked from anywhere else : it can appear in searches
https://scitools.org.uk/iris/docs/latest/whitepapers/change_management.html

We removed it with Iris 2 / dask development
#2862
This was because we then found that it promises too much on backwards compatibility.
Basically, enabling "dask behaviour" on a FUTURE was not a practical possibility.

TODO NOW: revise + reinstate or remove

? Comment, anyone ?

[mo-g]

Alternative third option: follow what you did with change_management_goals.rst and spin the content into other parts of the documentation. Maybe some of this has already happened? I haven't read the documentation comprehensively since er... 1.7. I'm sorry. I'll put it on the to-do list! 😞

The documentation it contains seems useful and relevant in and of itself, so I'd support some form of retention if it hasn't already been 'reimplemented'.

[pp-mo]

Done with #4217