fix: V2 docs / error messages (#3611)

* docs: rewrite concept section

* docs: add instance to guides

* chore: error messages

* fix: scenarios

* docs: urls

* docs: change images

* docs: change images

* docs: change images

Co-authored-by: Livio Amstutz <livio.a@gmail.com>

docs/sidebars.js

@@ -42,6 +42,7 @@ module.exports = {
       collapsed: false,
       items: [
         "guides/basics/get-started",
+        "guides/basics/instance",
         "guides/basics/organizations",
         "guides/basics/projects",
       ],
@@ -164,6 +165,8 @@ module.exports = {
             "apis/proto/auth",
             "apis/proto/management",
             "apis/proto/admin",
+            "apis/proto/system",
+            "apis/proto/instance",
             "apis/proto/org",
             "apis/proto/user",
             "apis/proto/app",
@@ -175,6 +178,7 @@ module.exports = {
             "apis/proto/metadata",
             "apis/proto/message",
             "apis/proto/text",
+            "apis/proto/action",
             "apis/proto/object",
             "apis/proto/options",
           ],
@@ -221,6 +225,7 @@ module.exports = {
       collapsed: true,
       items: [
         "concepts/structure/overview",
+        "concepts/structure/instance",
         "concepts/structure/organizations",
         "concepts/structure/policies",
         "concepts/structure/projects",