Docs: Add page about additional Postgres functionality

`NOW()` is a valid input for date* fields but this behavior is not documented anywhere at all. Is `NOW()` the only exception or are there other keywords/functions that can be used as well e.g. `CURRENT_DATE`, `CURRENT_TIME`, `LOCALTIME` or `NOW() at time zone 'UTC'`?

No documentation file or test is named in the issue. Start by locating the docs entry point for date fields and checking which PostgreSQL expressions Hasura accepts; done means documenting NOW() and any confirmed equivalents, with their scope and examples clearly stated.

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