docs(core): add how dom strategies work

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

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+# DomStrategy
+
+`DomStrategy` is a class exposed by @abp/ng.core package. Its instances define how an element will be attached to the DOM and are consumed by other classes such as `LoadingStrategy`.
+
+
+## API
+
+
+### constructor(public target?: HTMLElement, public position?: InsertPosition)
+
+`target` is an HTMLElement (_default: document.head_).
+`position` defines where the created element will be placed. All possible values of `position` can be found [here](https://developer.mozilla.org/en-US/docs/Web/API/Element/insertAdjacentElement) (_default: 'beforeend'_).
+
+
+### insertElement(element: HTMLElement): void
+
+This method inserts given `element` to `target` based on the `position`.
+
+
+
+
+## Predefined Dom Strategies
+
+Predefined dom strategies are accessible via `DOM_STRATEGY` constant.
+
+
+### AppendToBody()
+
+`insertElement` will place the given `element` at the end of `<body>`.
+
+
+### AppendToHead()
+
+`insertElement` will place the given `element` at the end of `<head>`.
+
+
+### PrependToHead()
+
+`insertElement` will place the given `element` at the beginning of `<head>`.
+
+
+### AfterElement(target: HTMLElement)
+
+`insertElement` will place the given `element` after (as a sibling to) the `target`.
+
+
+### BeforeElement(target: HTMLElement)
+
+`insertElement` will place the given `element` before (as a sibling to) the `target`.
+
+
+
+
+## What's Next?
+
+- [LoadingStrategy](./Loading-Strategy.md)