create a website for guides

[meleu]

I'm working on a website similar to what we have for RADocs, but for the guides we have here.

TODO:

• [x] adjust the wiki's Sidebar (use RAdocs-wiki as inspiration)
• [x] ask @luchaos to create the guides subdomain (CNAME here will be guides.retroachievements.org)
• [x] create an "edit link" similar to what we have in https://retroachievements.org/wiki-edit-redirect.php
• [ ] show to @ViperZang the peculiarities of markdown and how it differs from what we see in the wiki and in site created with mkdocs.

[luchaos]

✅ DNS entry & activate GitHub Pages domain http://guides.retroachievements.org/ ✅ Context aware edit links: https://github.com/RetroAchievements/RAWeb/pull/579