Re-organize support page as plain Markdown files

In our meeting on 10.09., we agreed to move the support website to GitLab itself, without involving a third party. Without GitLab Pages enabled, we will re-arrange the content to plain Markdown files.

• delete all Jekyll/Kramdown extensions
• fix links (after discussing content and structure)

mentioned in commit b898b02a

mentioned in commit e0d9ff07

changed the description

mentioned in commit 9a0bf2f2

marked the checklist item delete all Jekyll/Kramdown extensions as completed

marked the checklist item fix links (after discussing content and structure) as completed

closed