fix(content-docs): getMainDocId should return doc with both versioned or unversioned id

[slorber]

Motivation

Little retro compatibility issue after updating the sidebar versioning strategy (see also #6018)

Affecting the Jest website, which is not using slug: / unlike our website, so we didn't see this issue on our own website

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

local only, it's code that we'll remove quite soon anyway (as part of #6018)

[netlify]

✔️ [V2]

🔨 Explore the source changes: cd97e31

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61b380f53567890007b4e1d3

😎 Browse the preview: https://deploy-preview-6085--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	95
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-6085--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 652 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	37.8 kB
website/build/assets/css/styles.********.css	101 kB
website/build/assets/js/main.********.js	484 kB
website/build/index.html	29.3 kB

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