add tests for cross model forward references, add docs for processing forwardrefs, wip on refactoring queries into separate pages based on functionality

docs/queries/aggregations.md

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+# Aggregation functions
+
+`ormar` currently supports 2 aggregation functions:
+
+*  `count() -> int`
+*  `exists() -> bool`
+
+## count
+
+`count() -> int`
+
+Returns number of rows matching the given criteria (i.e. applied with `filter` and `exclude`)
+
+```python
+# returns count of rows in db for Books model
+no_of_books = await Book.objects.count()
+```
+
+## exists
+
+`exists() -> bool`
+
+Returns a bool value to confirm if there are rows matching the given criteria (applied with `filter` and `exclude`)
+
+```python
+# returns a boolean value if given row exists
+has_sample = await Book.objects.filter(title='Sample').exists()
+```