fix(core): normalize slashes for url/baseUrl instead of throwing

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes Site config url + baseUrl are not normalized and sensitive to leading/trailing slash #7989) and the maintainers have approved on my working plan.

Motivation

Test Plan

Testing the SiteMetadata is a bit unwieldy atm, so I only wrote validation tests.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	271e7a2
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/63196ea786d9490009680e84
😎 Deploy Preview	https://deploy-preview-8066--docusaurus-2.netlify.app
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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 82	🟢 98	🟢 100	🟢 100	🟠 80	Report
/docs/installation	🟠 76	🟢 100	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: 0 B

Total Size: 813 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.1 kB
website/build/assets/css/styles.********.css	111 kB
website/build/assets/js/main.********.js	609 kB
website/build/index.html	40.8 kB

_{compressed-size-action}

[slorber]

LGTM thanks 👍