Consistently use github markdown for alerts

[thompson-tomo]

Currently in the docs there are occasions where we are using GitHub flavored markdown for alert messages, however there are locations where we are instead using quote with bold text.

These non GFM alerts should be switched to improve styling especially given that OTEL.io is now able to stylize these alerts with color.

There are also other locations which are using block quote with symbol to indicate the alert type. These should be switched as well.

• [chore] move to gfm alerts also supported in hugo #3112
• update entity pages