Restructure grammar re shared modules

[toddjonker]

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          I would pull this [`shared-module-def`] out into a different section because it is not part of the domain model.  To wit, it is orphaned in the grammar.  At a minimum it should be noted that it is not formally part of the model, but I think it goes in a separate annex of the grammar alone with shared symbol table.

Originally posted by @almann in https://github.com/amazon-ion/ion-docs/pull/224#discussion_r1199294633