write numbers intro (#1183)
write numbers intro (#1183)

* write numbers intro

* lint fixes

* edit numbers intro

* add a little more back to the numbers intro

* Update concepts/numbers/introduction.md

Co-authored-by: Eli Flanagan <10695c13cf85d13812306a923912e4c6d13ce56c@users.noreply.github.com>

* Update concepts/numbers/introduction.md

* add code block ending

Co-authored-by: Eli Flanagan <10695c13cf85d13812306a923912e4c6d13ce56c@users.noreply.github.com>
exercism/xrust,exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,petertseng/exercism-rust