docs: Document "pre/post hooks"

[pawamoy]

#1733 adds an "operation" variable that can be used to decide whether to run tasks or migrations during copy or update. This resolves the initial request in #240, but not the added requests to have pre/post "stages" too.

After @sisp's comment on this, I suggest we close #240 when merging #1733, and add a documentation section like "Pre/post hooks", which is how Cookiecutter named them, and will be familiar to most users.

This entry would document that you can use migrations as pre/post-update hooks (with examples), tasks as post-copy or post-update hooks, and that... pre-copy hooks are not supported yet. The docs entry should emphasize that for simple message display, users can set message_after_copy and related settings.