Add Screen orientation lock instructions

[joegoodall1]

Instructions to lock screen orientations to either portrait or landscape for Android and iOS

Motivation

To help out with the Improving User Experience docs

Release Notes

[DOCS] [IOS] [ANDROID] [ENHANCEMENT] [ImprovingUserExperience.md] - Adding instructions for screen orientation locking for Android/iOS

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[pull-bot]

	Messages
📖	📄 Docs - Thanks for your contribution to the docs!

@facebook-github-bot label Documentation

Attention: @hramos

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[facebook-github-bot]

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[facebook-github-bot]

@joegoodall1 I tried to find reviewers for this pull request and wanted to ping them to take another look. However, based on the blame information for the files in this pull request I couldn't find any reviewers. This sometimes happens when the files in the pull request are new or don't exist on master anymore. Is this pull request still relevant? If yes could you please rebase? In case you know who has context on this code feel free to mention them in a comment (one person is fine). Thanks for reading and hope you will continue contributing to the project.

[hramos]

Thanks for the contribution. The docs have moved to their own repo at facebook/react-native-website. I've applied your change over there.