feat(theme-classic): make first tab the default tab

[Josh-Cena]

Motivation

It's quite weird UX to display no tabs if no default has been provided—how many users would want only headings with no content anyways? (This behavior is still achievable with <Tabs defaultValue="none">)

A further fix I can think of is to throw an error when defaultValue is non-existent, and use defaultValue={null} to specially indicate no tabs open by default. This helps to catch typos early. But that could be a minor BC. (The current fix is already a minor behavior inconsistency, but probably with little effect)

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Updated all the tabs usage on the website.

[netlify]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	91
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5647--docusaurus-2.netlify.app/

[slorber]

Thanks, also noticed that weird behavior 👍

A further fix I can think of is to throw an error when defaultValue is non-existent, and use defaultValue={null} to specially indicate no tabs open by default. This helps to catch typos early. But that could be a minor BC. (The current fix is already a minor behavior inconsistency, but probably with little effect)

Totally agree with this breaking change, we want to fail fast here and prevent user mistakes