docs: add how container strategies work

docs/en/UI/Angular/Container-Strategy.md

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+# ContainerStrategy
+
+`ContainerStrategy` is an abstract class exposed by @abp/ng.core package. There are two container strategies extending it: `ClearContainerStrategy` and `InsertIntoContainerStrategy`. Implementing the same methods and properties, both of these strategies help you define how your containers will be prepared and where your content will be projected.
+
+
+
+## API
+
+`ClearContainerStrategy` is a class that extends `ContainerStrategy`. It lets you **clear a container before projecting content in it**.
+
+
+### constructor
+
+```js
+constructor(
+  public containerRef: ViewContainerRef,
+  private index?: number, // works only in InsertIntoContainerStrategy
+)
+```
+
+- `containerRef` is the `ViewContainerRef` that will be used when projecting the content.
+
+
+### getIndex
+
+```js
+getIndex(): number
+```
+
+This method return the given index clamped by `0` and `length` of the `containerRef`. For strategies without an index, it returns `0`.
+
+
+### prepare
+
+```js
+prepare(): void
+```
+
+This method is called before content projection. Based on used container strategy, it either clears the container or does nothing (noop).
+
+
+
+## ClearContainerStrategy
+
+`ClearContainerStrategy` is a class that extends `ContainerStrategy`. It lets you **clear a container before projecting content in it**.
+
+
+
+## InsertIntoContainerStrategy
+
+`InsertIntoContainerStrategy` is a class that extends `ContainerStrategy`. It lets you **project your content at a specific node index in the container**.
+
+
+
+## Predefined Container Strategies
+
+Predefined container strategies are accessible via `CONTAINER_STRATEGY` constant.
+
+
+### Clear
+
+```js
+CONTAINER_STRATEGY.Clear(containerRef: ViewContainerRef)
+```
+
+Clears given container before content projection.
+
+
+### Append
+
+```js
+CONTAINER_STRATEGY.Append(containerRef: ViewContainerRef)
+```
+
+Projected content will be appended to the container.
+
+
+### Prepend
+
+```js
+CONTAINER_STRATEGY.Prepend(containerRef: ViewContainerRef)
+```
+
+Projected content will be prepended to the container.
+
+
+### Insert
+
+```js
+CONTAINER_STRATEGY.Insert(
+  containerRef: ViewContainerRef,
+  index: number,
+)
+```
+
+Projected content will be inserted into to the container at given index (clamped by `0` and `length` of the `containerRef`).
+
+
+## See Also
+
+- [ProjectionStrategy](./Projection-Strategy.md)