New ReactOS Website: Reorganize introductory pages

I am 120% sure the team is aware of this topic, I post this here to make sure it is being tracked.

Currently, the repository contains pages for newcomers, and there are some overlaps between them:

• installation.md and installing-reactos.md
• contributing.md, joining.md and participation.md (+ mailing-list.md?)
  (+ architecture.md? I guess that this one will be rewritten to describe the ReactOS architecture)

They probably would be:

• a page introducing communication channels: mailing lists, Mattermost, forum, ...,
• a page suggesting participation: as a developer, tester, artist, translator, ..., and
• the installation guide.

They don't need to be three; four or five may be fine as long as they have different roles.