Add economy page

[mdcfe]

Add economy page to:

• Explain how to use economy commands
• Explain how to use EssentialsX Economy with other plugins (Vault)
• Explain how to use an alternative economy plugin
• Explain different config options for currency

[mdcfe]

We could adapt the page from Essentials wiki, but it needs to include the new formatting config options. Additionally, the Register plugin has been dead for years so we can remove references to it (not to be confused with Reserve, which will be supported in EssentialsX soon).

The API page could also be migrated eventually, but as the Economy API is outdated and may not be around forever in its current form, I'm not too sure about this.