docs: reorganize actions v3 docs (#8712)

# Which Problems Are Solved

The actions v2 docs have broken links. Also, actions docs are
distributed and the navigation feels messy.

# How the Problems Are Solved

- The two actions v2 docs pages are integrated into the more accurate
actions v3 API section. Redirects are added to the Vercel config.


![image](https://github.com/user-attachments/assets/0b3b2255-4b90-45b3-be1d-ee875dd86f03)

![image](https://github.com/user-attachments/assets/a35314d7-11fb-446e-ae47-9d3e65adeccd)

- The broken links are fixed.

# Additional Changes

A text is added to the actions v3 section describing the changes coming
with v3.


![image](https://github.com/user-attachments/assets/bba0c9a2-36af-4820-aa84-a823b8801d79)

# Additional Context

- Replaces #8688
- Broken links reported in
https://discord.com/channels/927474939156643850/1275551972283646055
- Broken links reported in
https://discord.com/channels/927474939156643850/1168451861515354133/1280394234679136267

---------

Co-authored-by: Fabi <fabienne@zitadel.com>

docs/sidebars.js

@@ -745,10 +745,20 @@ module.exports = {
                 slug: "/apis/resources/action_service_v3",
                 description:
                   "This API is intended to manage custom executions and targets (previously known as actions) in a ZITADEL instance.\n" +
+                  "The version 3 of actions provide much more options to customize ZITADELs behaviour than previous action versions.\n" +
+                  "Also, v3 actions are available instance-wide, whereas previous actions had to be managed for each organization individually\n" +
+                  "ZITADEL doesn't restrict the implementation languages, tooling and runtime for v3 action executions anymore.\n" +
+                  "Instead, it calls external endpoints which are implemented and maintained by action v3 users.\n" +
                   "\n" +
                   "This project is in Preview state. It can AND will continue breaking until the services provide the same functionality as the current actions.",
               },
-              items: require("./docs/apis/resources/action_service_v3/sidebar.ts"),
+              items: [{
+                type: "doc",
+                id: "apis/actions/v3/usage",
+              }, {
+                type: "doc",
+                id: "apis/actions/v3/testing-locally",
+              }].concat(require("./docs/apis/resources/action_service_v3/sidebar.ts")),
             },
             {
               type: "category",
@@ -815,12 +825,6 @@ module.exports = {
         "apis/actions/objects",
       ],
     },
-    {
-      type: "category",
-      label: "Actions V2",
-      collapsed: false,
-      items: ["apis/actionsv2/introduction", "apis/actionsv2/execution-local"],
-    },
     {
       type: "doc",
       label: "gRPC Status Codes",