docs(core): add how cross-origin strategies work

docs/en/UI/Angular/Cross-Origin-Strategy.md

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+# CrossOriginStrategy
+
+`CrossOriginStrategy` is a class exposed by @abp/ng.core package. Its instances define how a source referenced by an element will be retrieved by the browser and are consumed by other classes such as `LoadingStrategy`.
+
+
+## API
+
+
+### constructor(public crossorigin: 'anonymous' | 'use-credentials', public integrity?: string)
+
+`crossorigin` is mapped to [the HTML attribute with the same name](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/crossorigin).
+`integrity` is a hash for validating a remote resource. Its use is explained [here](https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity).
+
+
+### setCrossOrigin(element: HTMLElement): void
+
+This method maps the aforementioned properties to the given `element`.
+
+
+
+
+## Predefined Cross-Origin Strategies
+
+Predefined cross-origin strategies are accessible via `CROSS_ORIGIN_STRATEGY` constant.
+
+
+### Anonymous(integrity?: string)
+
+`crossorigin` will be set as `"anonymous"` and `integrity` is optional.
+
+
+### UseCredentials(integrity?: string)
+
+`crossorigin` will be set as `"use-credentials"` and `integrity` is optional.
+
+
+
+
+## What's Next?
+
+- [LoadingStrategy](./Loading-Strategy.md)