docs: fixes DOC-30

[melissahenderson]

Description of changes

• Optimized image sizes in img/snippets.
• Deleted 4-sig-creation.png instead of redoing it. I don't think the image added any value.
• Revised the Signature Creation section of http-signatures.mdx to make it flow a bit better now that the image is removed.

Required

• [Y] Used LinkOut component on external links
• [Y] Reviewed Vale errors and made changes where appropriate
• [Y] Ran Prettier
• Previewed updates in Netlify
• [N/A] Received SME and/or peer approval if updates are significant
• [Y] Included a "fixes #" comment

Conditional

• Ensured sequence diagrams follow our style guide
• Included code samples where appropriate
• Updated related READMEs

[netlify]

✅ Deploy Preview for openpayments-preview ready!

Name	Link
🔨 Latest commit	597bd3a
🔍 Latest deploy log	https://app.netlify.com/projects/openpayments-preview/deploys/69c2daae60d10c0008d3aced
😎 Deploy Preview	https://deploy-preview-770--openpayments-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.