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
Renamed session_serializer option to cookies_serializer
Just very so slightly better test coverage
Added HybridSerializer to upgrade existing marshal cookies (wip: need tests)
Removed an old test
Tests for the HybridSerializer
Fixed minor typo in test code
Changed the tests to ensure HybridSerializer actually migrates the cookies (currently failing)
Convert FlashHash in a Hash with indifferent access
Stringify the incoming hash in FlashHash

Stringify the incoming as well to handle incoming symbol keys from
marshalled sessions
Re-write legacy (marshal) cookies on read
Remove serializer option from session_store.rb template
Fix AppGeneratorTest: serializer option was removed from session_store
Fixed broken flash tests
Missed FlashHash#replace
Added changelog entry for Flash changes [ci skip]
Updated CHANGELOG, docs, guides and release notes.

Also added a `cookies_serializer.rb` initializer to the app template.
Migrate hash-based cookie values correctly
rm warning about variable shadowing
Merge pull request #13571 from bak/getting_started_guide

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