Improve documentation

[JohnMertz]

Create and move documentation to GitHub wiki. This will be the new source of documentation going forward.

• [x] Introductory document: https://github.com/MailCleaner/MailCleaner-Next/wiki
• [x] Introduction to testing: https://github.com/MailCleaner/MailCleaner-Next/wiki/Alpha-Testing
• [x] Contribution guide: https://github.com/MailCleaner/MailCleaner-Next/wiki/Contributing-Guide
• [x] Testing installation guide: https://github.com/MailCleaner/MailCleaner-Next/wiki/Installation-Guide
• [ ] Source installation guide: to be copied from Testing guide above when VM images become available
• [ ] VM installation guide: to be created when images become available
• [ ] Migrate any documents that are still relevant from https://support.mailcleaner.net/projects/support-user/boards

Please comment on this issue with other specific documentation that you require. You can also contribute to the wiki directly.