We’re in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
Plugins can be divided in two groups. XUL plugins for Firefox or Zotero Standalone that closely integrate with the Zotero client, and external plugins, which are more likely to rely on the Server API to access Zotero libraries.
Existing (open source) plugins can be very helpful in helping you develop you own plugins (see the list of Zotero plugins). If you plan to write a XUL plugin, you can start by taking a look at the Hello World Zotero plugin.