CIVICRM-1707 Remove incorrect statement about open_basedir being deprecated. Safe mode was deprecated in PHP 5.3, so no longer relevant. Include minimum PHP configuration recommendations and links.

Update Requirements doc to mention that PHP7.4 is supported from MySQL 5.7 and also to clarify that all CMSes now supoort MySQL8 and add a note about installing on Cloud Service Providers such as AWS and Azure

The list of requirements is hard to take in at a glance. This adds a section that
you can read at a glance. It's obviously not as thorough as the main requirements,
so it's only presented as a summary-recommendation.

This repo currently has 16 markdown files, of which 8 have `CR` endings and
8 have `CR-LF` endings.  For comparison, the dev-docs and sysadmin-guide
have strictly `CR` endings.