Add example on chaining keyframes

[Joha99]

What changed / motivation ?

Please include relevant motivation and context

This PR adds an example on how to chain multiple keyframes in the keyframes docs.
Chaining multiple keyframes is very common and adding a doc on how to do it using stylex.keyframes would be a nice addition.

Linked PR/Issues

Fixes # (issue)

Related to this reported issue: #213

Additional Context

Screenshots, Tests, Anything Else

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

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[mellyeliu]

Changes look good! Please sign the CLA before we merge

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