[chore] move to gfm alerts also supported in hugo

[thompson-tomo]

Progresses #3111

Changes

This switches the docs from using bold text to indicate the alert type to instead using gfm alert which hugo can understand and stylize accordingly.

Comparison:

	Github	Website
Old
New

This also sets stability notes as important based on the definition from https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts

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Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [ ] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [ ] Links to the prototypes or existing instrumentations (when adding or changing conventions)