This commit contains an entirely new draft of the 
getting started docs

Following on from a round of feedback from several
reviewers, this commit contains minor changes to the
new Getting Started guide.

[+] Minor spelling / grammatical changes
[+] Changes to overall readability
[-] Remove "Composer Commands" section

This commit contains an entirely new draft of the 
getting started docs

Following on from a round of feedback from several
reviewers, this commit contains minor changes to the
new Getting Started guide.

[+] Minor spelling / grammatical changes
[+] Changes to overall readability
[-] Remove "Composer Commands" section

This commit breaks out the extension guide into two sections, one handles
composer based installations, the other is dedicated to the legacy installation
method.

[+] Create Index page with menu and cards
[+] Move content into the Composer guide
[+] Place remaining content in the legacy guide

This commit adds information explaining how local extensions are installed
and loaded via composer.

[+] Add card on index page
[+] Add section in Managing Extensions