Update URLs

Redirects are in place (or will be shortly). This is mostly
housekeeping.

Maven repo URLs already redirect however we can't deploy through them.
Jenkins URLs do not currently redirect but will within 24 hours.

Also adds a Discord link to the README, because apparently that's what
people want.

Documentation will be updated and announcements made once all redirects
are in place, until then, consider this advanced notice.

Spigot-API-Patches/POM-changes.patch

@@ -45,13 +45,13 @@ index 73ef3193..4220760f 100644
 -            <id>spigotmc-releases</id>
 -            <url>https://hub.spigotmc.org/nexus/content/repositories/releases/</url>
 +            <id>destroystokyo-releases</id>
-+            <url>https://repo.destroystokyo.com/repository/maven-releases/</url>
++            <url>https://destroystokyo.com/repo/repository/maven-releases/</url>
          </repository>
          <snapshotRepository>
 -            <id>spigotmc-snapshots</id>
 -            <url>https://hub.spigotmc.org/nexus/content/repositories/snapshots/</url>
 +            <id>destroystokyo-snapshots</id>
-+            <url>https://repo.destroystokyo.com/repository/maven-snapshots/</url>
++            <url>https://destroystokyo.com/repo/repository/maven-snapshots/</url>
          </snapshotRepository>
      </distributionManagement>