docs: update sparkpost.com/docs links to docs.sparkpost.com subdomain

[danielfsousa]

Summary

• SparkPost's docs site canonical host moved from www.sparkpost.com/docs to docs.sparkpost.com/docs. This PR updates the remaining outbound links in content/pages/index.js and src/components/Footer/index.js that still pointed at the old www.sparkpost.com/docs host.
• Mechanical www.sparkpost.com/docs → docs.sparkpost.com/docs substring replace; no other links touched.

Test plan

• Spot-check the homepage and footer links render correctly

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Note

Low Risk
Cosmetic URL-only changes to external documentation links with no application logic or data handling impact.

Overview
Updates outbound documentation URLs from www.sparkpost.com/docs to docs.sparkpost.com/docs so links match the docs site’s canonical host.

On the homepage (content/pages/index.js), the getting-started full guide link and two step links (domain setup and first send) now use the subdomain; anchor paths are unchanged. The footer Help & Docs item (src/components/Footer/index.js) uses the same host swap. No other links or behavior were modified.

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[danielfsousa]

Checked all 4 links in this PR (root docs link, getting-started-sparkpost/ plus its #preparing-your-from-address and #sending-email anchors) against the live site — all resolve with 200 and the anchors exist on the target page. No changes needed here.

[brandonglesmann]

LGTM