Refactor source install overview

[joshtrichards]

☑️ Resolves

Started as a simple change to clarify the confusing paragraph about "Nextcloud comes with all the required PHP modules". Ended up doing some refactoring and enhancements of the first part of this section because I couldn't help myself. :-)

• Tidying / ordering of the existing material
• Tried to clarify various bits
• Extended info about the various installation methods

P.S. As with anything involving writing, chances are I'll find numerous typos myself tomorrow, but don't hesitate to point out anything you find too.

🖼️ Screenshots