DOP-5038: add segment tracking to dark mode controls

[seungpark]

Stories/Links:

DOP-5038

Current Behavior:

docs landing without tracking

Staging Links:

staging link with user tracking

Notes:

• To test, you can visit the live debugger for segment actions and visit the above staging link to test the analytics:

README updates

• • This PR introduces changes that should be reflected in the README, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README

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