Remove all external plugin docs

This change removes all external plugin docs using the old remote docs framework
from the Packer core documentation in favor of the Packer integration framework.
Remove plugins will be enabled on the integration portal and contacted to merge their PRs to finalize the integration migration.

website/data/plugins-manifest.json

@@ -1,99 +1,2 @@
 [
-  {
-    "title": "Anka",
-    "path": "anka",
-    "repo": "veertuinc/packer-plugin-veertu-anka",
-    "pluginTier": "community",
-    "sourceBranch": "master",
-    "version": "latest"
-  },
-  {
-    "title": "DigitalOcean",
-    "path": "digitalocean",
-    "repo": "digitalocean/packer-plugin-digitalocean",
-    "pluginTier": "verified",
-    "version": "latest",
-    "isHcpPackerReady": true
-  },
-  {
-    "title": "External",
-    "path": "external",
-    "repo": "joomcode/packer-plugin-external",
-    "version": "latest",
-    "sourceBranch": "main"
-  },
-  {
-    "title": "Gridscale",
-    "path": "gridscale",
-    "repo": "gridscale/packer-plugin-gridscale",
-    "version": "latest",
-    "pluginTier": "verified",
-    "isHcpPackerReady": false
-  },
-  {
-    "title": "IPSW",
-    "path": "ipsw",
-    "repo": "torarnv/packer-plugin-ipsw",
-    "version": "latest",
-    "sourceBranch": "main",
-    "pluginTier": "community"
-  },
-  {
-    "title": "Kamatera",
-    "path": "kamatera",
-    "repo": "kamatera/packer-plugin-kamatera",
-    "pluginTier": "community",
-    "version": "latest"
-  },
-  {
-    "title": "Libvirt",
-    "path": "libvirt",
-    "repo": "thomasklein94/packer-plugin-libvirt",
-    "pluginTier": "community",
-    "version": "latest"
-  },
-  {
-    "title": "Nutanix",
-    "path": "nutanix",
-    "repo": "nutanix-cloud-native/packer-plugin-nutanix",
-    "version": "latest",
-    "pluginTier": "verified",
-    "sourceBranch": "main"
-  },
-  {
-    "title": "Outscale",
-    "path": "outscale",
-    "repo": "outscale/packer-plugin-outscale",
-    "version": "latest",
-    "pluginTier": "verified",
-    "isHcpPackerReady": true
-  },
-  {
-    "title": "UCloud",
-    "path": "ucloud",
-    "repo": "ucloud/packer-plugin-ucloud",
-    "version": "latest",
-    "pluginTier": "community"
-  },
-  {
-    "title": "Volcengine",
-    "path": "volcengine",
-    "repo": "volcengine/packer-plugin-volcengine",
-    "pluginTier": "community",
-    "version": "latest"
-  },
-  {
-    "title": "Vultr",
-    "path": "vultr",
-    "repo": "vultr/packer-plugin-vultr",
-    "pluginTier": "community",
-    "version": "latest"
-  },
-  {
-    "title": "Ksyun",
-    "path": "ksyun",
-    "repo": "kingsoftcloud/packer-plugin-ksyun",
-    "pluginTier": "community",
-    "version": "latest"
-  }
 ]