Add more documentation/examples about bypass_teams option

With the upcoming "Updated GitHub repository security recommendations and actions for ASF projects" and given that by default I see that the .asf.yaml snippet contains the following entry
```
bypass_teams:
- root
```
I would like to understand more about this option.

First of all I would like to confirm that this group can bypass the defined rulesets. In other words, are people in this group capable of force-pushing and restoring a branch to an arbitrary state?

Secondly, I would like to know what does `root` mean, and which people/team are part of it. If problems arise and we want to contact them to perform a force-push what's the recommended path forward.

Finally, can we have form arbitrary groups of people that have these special bypass permissions? Is it possible for instance to just have the PMC chair of the project capable of bypassing the rulesets?

Start with the `.asf.yaml` `bypass_teams` entry and review how GitHub repository rulesets define bypass permissions. Document what `root` represents, how to contact it, whether custom groups such as a PMC chair can be configured, and add examples that answer the questions in the issue.

Written by the indexing model from the issue text.