[Cedar Doc Issue] Add examples for using Regex in Policies

[apandays]

What were you trying to do?
Writing an regex to check starts with

What is wrong and why?
I could not figure out from the docs how to make it work. The policy grammar sections is technically correct, but very difficult to use. it is also very difficult to find, i did not realize that the operators would be in the policy grammer section.

What do we need to do to fix this?

1. Update the section name to be more colloquial
2. Provide examples for all the operators, especially the regex
3. Add common usecases for the regex and add examples

[AMZ-brandon]

Thanks, I'll look into this.