To add to nixer55's answer to the first question:
Yes, it's custom HTML, "Hosted on GitHub Pages using the Jekyll server, Markdown formatting, Liquid templates, and YAML data format." Its source is available in the two GitHub repositories mybb/mybb.com and mybb/docs.mybb.com. Any old HTML/PHP generated by any method, including manually, would work just as well though - so, no, a CMS isn't required, and could even introduce extra difficulties in having to synchronise user accounts between the CMS and MyBB if that were to be a requirement.
There's also a team-only GitHub repository for the Extend section of the site, which is a MyBB plugin.
Yes, it's custom HTML, "Hosted on GitHub Pages using the Jekyll server, Markdown formatting, Liquid templates, and YAML data format." Its source is available in the two GitHub repositories mybb/mybb.com and mybb/docs.mybb.com. Any old HTML/PHP generated by any method, including manually, would work just as well though - so, no, a CMS isn't required, and could even introduce extra difficulties in having to synchronise user accounts between the CMS and MyBB if that were to be a requirement.
There's also a team-only GitHub repository for the Extend section of the site, which is a MyBB plugin.