[Task] Create Installation Guide in User Guide

[ericbsd]

• [x] This new file should be documentation/source/user/installation-guide/installation-guide.md
• [x] documentation/source/index.md need to be updated to have the new file under User Guide
• [x] It should contain a short introduction of GhostBSD and what the Installation Guide covers similar to the Introduction at https://wiki.ghostbsd.org/index.php/Installation_Guide#Introduction