Dropping Git clone and archive installation support

[macintoshplus]

We can install Bolt in 3 ways.

First, the extraordinary ways (for not say bad):

1. Use git clone https://github.com/bolt/core.
2. Download the version or branch archive and use it

Why? The code of the project is melting in the Bolt's code. The Bolt upgrade way is very difficult. The project can easily change the Bolt code.
How to know if the bug reported is from the original Bolt code or if the bug is due to a project Bolt code customizing?

The recommended and the current way in PHP development:

3. Create a project with composer create-project. Composer helps to manage the dependencies upgrade. The project code is not in another Bolt code.

Of course, we can discuss it in the comments.