Document citus.shard_placement_policy

[begriffs]

Why are we implementing it? (sales eng)

What are the typical use cases?

Communication goals (e.g. detailed howto vs orientation)

Good locations for content in docs structure

How does this work? (devs)

Example sql

Corner cases, gotchas

Are there relevant blog posts or outside documentation about the concept/feature?

Link to relevant commits and regression tests if applicable

[begriffs]

Interestingly there's a comment inside our reference section:

.. Comment out this configuration as currently COPY only support random
   placement policy.
.. citus.shard_placement_policy (enum)
   $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

[begriffs]

This requires more investigation in the code to see where/what it's used for.