RFR: Cleaning up headers and examples [v2]

[Jesper Wilhelmsson]

On Thu, 1 Sep 2022 15:40:27 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> * Make all headers follow the same style
>> * Make code examples use ~~~ instead of four space indentation
>> * Fix email adresses to be actual adresses
>> * Add link to jtreg FAQ
>> * Add comment about the name space for noreg-labels
>
> Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update src/guide/mailing-lists.md
>   
>   Co-authored-by: Alexey Ivanov <70774172+aivanov-jdk at users.noreply.github.com>

@aivanov-jdk Sorry, I didn't see your comments before integrating. I tried to remove the /integrate comment, but the bots were unusually fast today so they saw it anyway. I'll include your changes in my next patch.

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PR: https://git.openjdk.org/guide/pull/90