[6.x] Documentation, whats new, migration guide ...etc

[aedart]

The version 6.x series of the documentation must be improved. Among the known tasks to be done are:

• Write what's new in 6.x
• Migration Guide
• Re-write Core documentation (no longer a package to be used for upg. legacy applications...)
• Examples should be formatted acc. to PHP 8
• Security Policy (GitHub)
• Re-write Contribution guide (GitHub)

[aedart]

The vast majority of the documentation is in place. Now awaiting codeception & phpunit before able to release