Docs style: Alert boxes: Eliminate everything except Note and Warning

Problem to solve

In the docs we use alert boxes to draw reader's attention to text.

We currently have four main types: Note, Caution, Danger, Tip

We currently use the format: NOTE: **Note:**

Decision

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)

Work to do

To make this happen, we need to:

• Convert CAUTION: to WARNING: gitlab-docs#863 (closed).
• Remove extra text (extra **Note:**, **Warning:**, etc.) from docs: !44327 (merged)
• Remove non-standard bold text from docs: #244970 (closed)
• Update the Vale rules
• Update the style guide