Update the Getting Started tutorial

Update the Getting Started tutorial to clarify where and how to add
controller actions, and fix some grammar.

This PR is an improved version of #14828, #14822 and #14815.

[skip ci]
cleanup, remove trailing whitespace. [ci skip]
Merge pull request #14905 from jonatack/patch-7

Update the Getting Started tutorial
rewrites the API docs of favicon_link_tag [ci skip]

Motivated by #14907.