Document PGM usage on modern Minecraft versions

[TheRealPear]

Since June, PGM has been able to run on Paper 1.21. So far, there has been a few things in the docs that made this obvious (min-server-version and max-server-version). I know this is technically not stable yet, but I think it would be worth covering this more.

• Update server setup guide to recommend Paper 1.21 alongside SportPaper 1.8
• How do we handle Material Finder if it's only 1.8 items with its internal names? Similarly, what about the rest of reference pages?
• Related to Add more modern map examples #192, add some examples built in modern versions