doc: add doc on aggregations (#16144)

docs/aggregation.md

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+# Aggregations
+
+We will cover internal implementation of common aggregations in this document.
+
+
+## Frontend
+
+TODO
+
+## Expression Framework
+
+TODO
+
+## HashAggExecutor
+
+![aggregation components](./images/aggregation/agg-components.png)
+
+Within the `HashAggExecutor`, there are 4 main components:
+1. AggCalls.
+2. AggState.
+3. AggGroups.
+4. Persisted State.
+
+AggCalls are the aggregation calls for the query. For instance `SUM(v1)`, `COUNT(v2)` has the AggCalls `SUM` and `COUNT`.
+
+AggState is the state we use to compute to the result (output) of the aggregation call.
+Within each aggregation group, it will have an AggState for each AggCall.
+
+AggGroups are created per aggregation group.
+For instance with `GROUP BY x1, x2`, there will be a group for each unique combination of `x1` and `x2`.
+
+Whenever stream chunks come in, the executor will update the aggregation state for each group, per agg call.
+
+On barrier, we will persist the in-memory states.
+For `value` type aggregations, we will persist the state to the intermediate state table.
+This state table will store all value aggregations per group on a single row.
+
+For `MaterializedInput` type aggregations, these require tracking input state. For example, non-append-only min/max.
+For each of these aggregations, they have 1 state table (`AggStateStorage::MaterializedInput`) each. Within the state table, it will store the input state for each group.
+
+### Initialization of `AggGroups`
+
+![init-agg-group](./images/aggregation/init-agg-group.png)
+
+AggGroups are initialized when corresponding aggregation groups are not found in `AggGroupCache`.
+This could be either because the `AggGroupCache` got evicted,
+or its a new group key.
+
+It could take a while to initialize agg groups, hence we cache them in `AggGroupCache`.