A pass over the Getting Started guide sections 1, 2, and 3 [ci skip]

* Add note about managing Ruby environments
* Point to curated lists of Ruby learning resources
* Expound on DRY and Convention over Configuration
* Remove note on superuser--thankfuly coders new to Rails don't have to sudo anymore :)
* Installation/verification instructions for SQLite3, since it's a dependency
Merge pull request #13571 from bak/getting_started_guide

A pass over the Getting Started guide sections 1, 2, and 3 [ci skip]