WIP redesign notes and admonitions

[thaJeztah]

follow-up to #11640 (changes are in the last commit). It's still a WIP, but I had this branch lingering around on my machine as part of other changes, so I thought to push it as a PR to share.

This brings the initial work to redesign our notes and admonitions.

The following adminitions are supported:

• info: generic note with "info" icon
• tip: note with a "lightbulb" icon
• check: note with a "check" icon
• important: note with an exclamation mark
• warning: note with an exclamation mark in a triangle

There are some alternative icons that I considered (currently commented in the style-sheet, but to be removed)

Things remaining:

• update the examples and descriptions in "test.md"
• tweak line-heights / typography
• review uses of "warning" versus "important"; their purpose needs to be defined better (and styled accordingly)
• color-schemes: we can likely use the same colors for "dark" and "light" mode. There's also some colors ("warning", e.g.) that use some tweaking.
• add classes to existing notes to switch between a "regular" note to notes with an icon ("admonition").
• removing "note" as first line: when using an icon, the icon can replace "note" in many cases; removing the "note" makes the note more compact without loosing meaning, so we should remove those.
• for other notes, we should do the reverse: replace "note" with a more suitable introduction. I updated some pages to illustrate this. Setting a more descriptive first line can give notes a better context.

Examples

Here's some pages that could be used to view the updates styles:

• https://5f9818658b67be00091dbe36--docsdocker.netlify.app/engine/security/https/
• https://5f9818658b67be00091dbe36--docsdocker.netlify.app/engine/install/ubuntu/

Screenshots

And some screenshots

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[thaJeztah]

/cc @usha-mandya we can look at this one in our call later

[docker-robott]

Thanks for the pull request. We'd like to make our product docs better, but haven’t been able to review all the suggestions.
As our docs have also diverged, we do not have the bandwidth to review and rebase old pull requests.

If the updates are still relevant, review our contribution guidelines and rebase your pull request against the latest version of the docs, then mark it as fresh with a /remove-lifecycle stale comment.
If not, this pull request will be closed in 30 days. This helps our maintainers focus on the active pull requests.

Prevent pull requests from auto-closing with a /lifecycle frozen comment.

/lifecycle stale