docs(js): Update quick start guide splitlayout for Solid, SolidStart, Svelte

[inventarSarah]

DESCRIBE YOUR PR

This branch updates the following quick start guides to use the new split layout:

• Solid
• SolidStart
• Svelte

Closes: #17449

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[sentry]

Bug: The ## Next Steps heading in the Svelte guide is unintentionally auto-numbered by the StepConnector component, disrupting the document's flow.
_{Severity: LOW}

Suggested Fix

To prevent numbering, add the data-no-number attribute to the h2 element for "Next Steps". Alternatively, restructure the document to move the heading outside the StepConnector's scope, for example by using an <Expandable title="Next Steps"> component without a preceding h2.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: docs/platforms/javascript/guides/svelte/index.mdx#L331-L334

Potential issue: In the Svelte guide, the `StepConnector` component automatically
numbers all `h2` headings within its scope. The `## Next Steps` heading is an `h2`
element placed inside the `StepConnector`, which will cause it to be unintentionally
numbered as part of the setup guide. This disrupts the visual flow of the documentation
and misrepresents the "Next Steps" section as a required step, which is inconsistent
with other guides updated in this pull request.

_{Did we get this right? 👍 / 👎 to inform future reviews.}