feat: package manager & installable packages

[SychO9]

putting here for use in discuss

[luceos]

The docs seem to imply the extension manager is already installed. Nowhere is mentioned how to install this manager in case it isn't installed, eg on existing systems. This might alleviate some support questions.

[GreXXL]

The docs seem to imply the extension manager is already installed. Nowhere is mentioned how to install this manager in case it isn't installed, eg on existing systems. This might alleviate some support questions.

In the downloaded package it's already bundled. I don't know if we also changed the skeleton when you manually install with composer so that it defaults installs as a core extension. In that case it would be fine to describe it "as already installed"?

[SychO9]

I don't actually think we should bundle it for those installing from the CLI, but instead include the installation instructions of the extension manager i the section of installing flarum from the CLI, that way it's clear that if you wish to manage extensions from the interface (even though you are currently using the CLI to install) then here it is and how to add it.

Also need to add the extension's installation instructions in the extensions section like daniel mentions.