Modernize Installation Instructions

[AugustMiller]

Description

New users should use DDEV. Legacy installation methods are antithetical to learning how Craft works—in particular, updates and plugin installations.

The installation instructions now more closely resemble the quick-start guide on the starter project. Other info has been consolidated, eliminated, or moved into the Knowledge Base:

• Using the Starter Project now contains platform-agnostic instructions for using composer create-project
• Setting up a Craft Project from Scratch is where we talk about the lower-level concerns, vis à vis .htaccess, index.php, bootstrapping, web roots, etc.
• The multi-step setup tour has been moved to First-Time Setup

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