Add example docs for user menu component

[komal-eaton]

Fixes # .

Changes proposed in this Pull Request:

• Add the example docs

• Follow the React docs as appropriate

• Configure codeblocks using PrismJS

• Configure examples to run live using RN Web

Screenshots / Screen Recording (if applicable)

To Test:

• yarn prettier
• yarn docs-start

Any specific feedback you are looking for?

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