Create new admonition for rational

[kersten1]

Describe your idea.
Create a custom admonition for "rationale" for priv and unpriv. Needs a new icon.

Describe why it is important
riscv/riscv-isa-manual#1547

Additional context
New admonition needs to be agreed upon before implementing.

[wmat]

What about using the Font Awesome gear icon for a rationale admonition?

https://fontawesome.com/icons/gears?s=solid

[kersten1]

Looks cool! I was thinking the risc-v logo but that might not be good all smooshed down. I think we could use almost anything as long as we add it to a section in the beginning that explains what they all mean. And then add tags to the sections as we go too so we can exclude them, if needed.