docs: fix homepage test pages link

[Lovlace777]

Summary

• Fixes the homepage link reported in [🐞] Website: broken link #716 by replacing the broken /tests/ docs link with the repository's tests directory.
• Updates the sentence so it no longer describes the removed public route as live demos.
• Adds an empty changeset for the docs-only change.

Validation

• curl.exe -L -I https://partytown.qwik.dev/tests/ returned 404 Not Found, confirming the reported broken route.
• curl.exe -L -I https://github.com/QwikDev/partytown/tree/main/tests returned 200 OK.
• Select-String -Path docs\src\routes\index.mdx -Pattern '\](/tests/)' returned no matches.
• git diff --check passed with line-ending warnings only from the Windows checkout.
• git diff --cached --check passed before commit.
• npx --yes prettier@3.6.2 --check .changeset/fix-tests-doc-link.md passed.

Notes

• Fixes [🐞] Website: broken link #716.
• No runtime behavior changes.
• Full docs build was not run locally because this checkout was narrowed with sparse checkout after repeated GitHub HTTPS clone interruptions; the change is limited to one docs link and its changeset.
• npx --yes prettier@3.6.2 --check docs/src/routes/index.mdx .changeset/fix-tests-doc-link.md would reformat pre-existing HTML anchor wrapping in the page, so I kept the diff limited to the broken link fix.

[changeset-bot]

🦋 Changeset detected

Latest commit: abc39fb

The changes in this PR will be included in the next version bump.

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[gioboa]

Thanks for your help @Lovlace777