Add custom Markdown container for API calls

[pwizla]

What does it do?

• Adds a responsive ::: api-call custom Markdown container and its ::: request and ::: response subcontainers
• Applies these everywhere in the docs

Why is it needed?

In preparation for v4 docs, we're improving our VuePress stack with custom elements.

These custom Markdown containers will make the API-related docs more enjoyable with:

• request and response visible at the same time
• a proper visual identity for API calls, different from the tabs we use for other purposes

Screen.Recording.2021-07-07.at.19.19.17.mov

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Notes

Design is still a work in progress. I received some feedback from our designers and will improve it.

To do:

• test other color palettes
  • add some "Strapi touch" to the color palette :-)
• check contrast ratio with tools like this one

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✅ Preview: https://documentation-git-repo-api-enhancements-strapijs.vercel.app

[pwizla]

Closing in favor of #368