Docs style: Alert boxes: Eliminate everything except Note and Warning
### Problem to solve In the docs we have a large number of [alert boxes](https://docs.gitlab.com/ee/development/documentation/styleguide/index.html#alert-boxes) that we use to draw readers' attention to text. These alert boxes have proliferated because it's easier and faster to add notes than to fit the information into existing content. When everything is accentuated, it's hard to determine what's really important. Sid also mentioned this in a recent docs review. Notes from his review are captured here: https://gitlab.com/groups/gitlab-org/-/epics/4527 We have a larger effort to [cut down on the number of alert boxes](https://gitlab.com/gitlab-org/technical-writing/-/issues/255) in the docs. This purpose of this epic is to remove extra text and standardize on two types of alerts. ### Decision We currently have four main types: Note, Caution, Danger, Tip, and we use the format: `NOTE: **Note:**`. We have decided to: - Show only the icons and no text. - Use only `NOTE:` and `WARNING:`. When you write `NOTE:` or `WARNING:` in a markdown file, an alert box is drawn around the text on the line below. The format of these alert boxes should be: * `NOTE:` should stay styled as it is today (pale gray and the `information-o` SVG icon) * `WARNING:` should be the style of danger today (pale red with `warning` SVG icon) The generated output includes text for screen readers. ### Work to do To make this happen, we need to: - Convert `CAUTION:` to `WARNING:` - Remove extra text (extra `**Note:**`, `**Warning:**`, etc.) from docs - Remove non-standard bold text from docs - Update the Vale rules - Update the style guide **The issues listed in this epic(s) are to be closed serially.**
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