Docs : use updated markdown for significant content

[romainmenke]

see : https://github.blog/changelog/2023-12-14-new-markdown-extension-alerts-provide-distinctive-styling-for-significant-content/

• markdown alerts : parallel builds notice #1214
• Update warning and caution messages #1215

[romainmenke]

I've added a few of these to improve some docs.

I don't find it easy to always pick the correct "level" but I don't think it's that important to have a rigid/consistent system for these.

Hopefully these improve readability :)