fix(theme-common): allow details to not provide a summary

[Josh-Cena]

Motivation

Right now, <details> without <summary> has a glitched UX:

Not only does it show the unstyled triangle, the content is not expandable either, because there's no area that listens to the click event.

The actual fallback string is user-agent defined, but "Details" seems to be canonically used (in Chrome and Safari), so let's use it as well. It doesn't have to be localized.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

It now shows up as clickable & expandable.

[netlify]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	57
🟢 Accessibility	100
🟠 Best practices	83
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-7267--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 803 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	50.1 kB
website/build/assets/css/styles.********.css	107 kB
website/build/assets/js/main.********.js	607 kB
website/build/index.html	38.8 kB

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