QuickStart notebook improvements

[nrichers]

Internal Notes for Reviewers

This PR includes some minor tweaks to the QuickStart notebook:

• Fix CTA callout for signups, now uses HTML that always renders correctly
• Add a “Get your code snippet” heading that makes it easier to spot where you need to go
• Remove duplicate TOC anchors in headings

External Release Notes

[cachafla]

Great 👍

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