refactor(website): polish on Showcase page

[slorber]

Motivation

Follow-up on #5742

• co-locate showcase comps close to page
• fix toggle shadow issue
• move users validation to Jest test
• add test for image size
• replace many screenshots

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests

[netlify]

✔️ [V2]

🔨 Explore the source changes: 6b670fe

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/6199247d3cb7af0008dc5cc3

😎 Browse the preview: https://deploy-preview-5969--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	77
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5969--docusaurus-2.netlify.app/

[github-actions]

Size Change: +56 B (0%)

Total Size: 890 kB

Filename	Size	Change
website/build/assets/css/styles.********.css	100 kB	+45 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	38.3 kB	0 B
website/build/assets/js/main.********.js	476 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	66.1 kB	0 B
website/build/blog/index.html	36.8 kB	0 B
website/build/docs/index.html	44 kB	+7 B (0%)
website/build/docs/installation/index.html	51.7 kB	+4 B (0%)
website/build/index.html	29.5 kB	0 B
website/build/tests/docs/index.html	25.2 kB	0 B
website/build/tests/docs/standalone/index.html	21.7 kB	0 B

_{compressed-size-action}

[Josh-Cena]

I doubt if the subdued separator is visible enough...

I have a feeling that if we unify the height of all images, using the primary color separator is going to show a better visual consistency

[slorber]

For me, the goal of that separator is not to be the most visible, just to be visible enough so that it's clear there's a separation, in case of using an image with a white background on a card with a white background too.

This does not look so great to me:

Going to merge now but open to iterate on this design again if people think we can do better