Improvements to install process for clarity and to match install docs

[DNA-styx]

Description

1. Provide clickable link from root to install folder after upload
2. Create check for empty config.php file which is created empty if the install process is started but not completed.
3. Updated required version numbers and checks during install
4. Added missing word to page three of install
5. Added cache folder with .gitkeep folder that is needed during install and not mentioned in install docs

Motivation and Context

Fixes common issues or confusions seen in discord support channels

How Has This Been Tested?

Created new SB++ site, ran through install process multiple times resetting files and database each time.

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.

[Rushaway]

For me, their is a reason why the installer is semi-hidden and not showed as link. But you are right on the update of requierements during install (php..)