[docs] Add header badge for PR staging instances

[cee-chen]

Summary

It occurred to me on a whim today that our PR staging instances (e.g. https://eui.elastic.co/pr_7347) should likely show some affordance for what they are (a staging server) rather than using the plain old version switcher (#7298). Here's my proposal for the different environments:

Staging (link goes back to the originating GitHub PR)

Local (no link/switcher)

Production (version switcher)

QA

Check it out! https://eui.elastic.co/pr_7347/#/

General checklist

N/A, docs only

[cee-chen]

@elastic/eui-team let me know what you think! Personally I think this is pretty useful (especially for demos or external links to a PR staging server where people may want to go to the original PR for more context).

Also looking for feedback on the badge color, is hollow too boring? Maybe we should use primary instead? :)

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_7347/
• Storybook: https://eui.elastic.co/pr_7347/storybook

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: b4a1fa0

History

• 💚 Build #995 succeeded 507c0f7
• 💚 Build #994 succeeded b588ccf

[1Copenut]

🚀 Wow, this is cool @cee-chen! I love the Staging link back to the PR, excellent touch.

[cee-chen]

Thanks Trevor!!

Going to go ahead and merge this so I can see it in action in a couple new/open PRs. But as always, if anyone has any feedback after the fact, just leave a comment - I'm always happy to open a follow-up PR or continue to tweak!