docs: add mutation example

docs/content/1.get-started/3.tips/2.server-side-calls.md

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+---
+title: Server Side Calls
+---
+
+# Server Side Calls
+
+You may need to call your procedure(s) directly from the server, `createCaller()` function returns you an instance of `RouterCaller` able to execute queries and mutations.
+
+## Input query example
+
+We create the router with a input query and then we call the asynchronous `greeting` procedure to get the result.
+
+::code-group
+
+```ts [server/trpc/trpc.ts]
+import { initTRPC } from '@trpc/server'
+import { z } from 'zod'
+ 
+const t = initTRPC.create()
+ 
+export const router = t.router({
+  // Create procedure at path 'greeting'
+  greeting: t.procedure
+    .input(z.object({ name: z.string() }))
+    .query(({ input }) => `Hello ${input.name}`),
+})
+```
+
+```ts [server/api/greeting.post.ts]
+import { caller } from '@/server/trpc/trpc'
+
+export default eventHandler(async (event) => {
+  const body = await getBody(event)
+  const caller = router.createCaller({})
+
+  const greeting = await caller.greeting({ name: body.name })
+
+  return {
+    greeting
+  }
+})
+```
+
+::
+
+## Mutation example
+
+We create the router with a mutation and then we call the asynchronous `post` procedure to get the result.
+
+::code-group
+
+```ts [server/trpc/trpc.ts]
+import { initTRPC } from '@trpc/server'
+import { z } from 'zod'
+ 
+const posts = ['One', 'Two', 'Three']
+ 
+const t = initTRPC.create()
+const router = t.router({
+  post: t.router({
+    add: t.procedure.input(z.string()).mutation(({ input }) => {
+      posts.push(input)
+      return posts
+    }),
+  }),
+})
+```
+
+```ts [server/api/post.post.ts]
+import { caller } from '@/server/trpc/trpc'
+
+export default eventHandler(async (event) => {
+  const body = await getBody(event)
+  const caller = router.createCaller({})
+
+  const post = await caller.post.add(body.post)
+
+  return {
+    post
+  }
+})
+```
+
+::