More detailed Ghost Record docs

[DVAlexHiggs]

Currently, we only have this:
https://dbtvault.readthedocs.io/en/latest/macros/#ghost-records
(link will fail until 0.9.1 docs are deployed)

This should be expanded with what GRs actually are, how we've implemented it (e.g. explaining how hashes, dates and payload columns work), with some examples