Remove github wiki to avoid confusion

[sebkuip]

Is your feature request related to a problem? Please elaborate.

People get confused about the github wiki, which is severely outdated. Instead, link to the wiki website.

Describe the solution you'd like

Remove github wiki and link to the outside wiki.

Does your solution involve any of the following?

• Logviewer
• New config option

Describe alternatives you've considered

N/A

Who will this benefit?

Newer users.

Additional Information

https://docs.modmail.dev/

[Taaku18]

Done, I've updated the README with the appropriate changes and removed the old wiki page.