[Doc] revise the update footer tutorial

[jcalcaben]

Description

Revise and publish the Update Footer tutorial contributed by @rossmc

Related Issue

Closes PWA-302

Acceptance

Any developer

Verification Stakeholders

Any developer

Specification

Verification Steps

1. navigate to the pwa-devdocs directory: cd pwa-devdocs
2. Run linter on file: yarn lint src/tutorials/pwa-studio-fundamentals/modify-site-footer
3. Verify no linting issues
4. Run preview server: yarn develop
5. Navigate to HTML preview site and go to the Tutorials section
6. Verify item appears on left navigation and links to the correct page
7. Verify page renders and content is accurate

Screenshots / Screen Captures (if appropriate)

Checklist

• I have updated the documentation accordingly, if necessary.
• I have added tests to cover my changes, if necessary.

[PWAStudioBot]

	Messages
📖	Access a deployed version of this PR here. Make sure to wait for the "pwa-pull-request-deploy" job to complete.
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).
📖	Associated JIRA tickets: PWA-302.

Generated by 🚫 dangerJS against dafb5cb

[larsroettig]

Looks good to me from the technical side