RFR: Cleaning up headers and examples [v2]

[Jesper Wilhelmsson]

> * 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>

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Changes:
  - all: https://git.openjdk.org/guide/pull/90/files
  - new: https://git.openjdk.org/guide/pull/90/files/524499bb..ce8fd5fe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=guide&pr=90&range=01
 - incr: https://webrevs.openjdk.org/?repo=guide&pr=90&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/90.diff
  Fetch: git fetch https://git.openjdk.org/guide pull/90/head:pull/90

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