Explicitly choose between config merging and clobbering configuration

[agoose77]

At present, it seems from conversations with @rowanc1 and @fwkoch that we're selecting an extension policy for each particular key in myst.yml.

I wonder if it would be nice to have a user-facing control mechanism for this, e.g.

site:
   options:
      extend: merge

I don't think this should be done at the config level, but rather per config-section. Thoughts?