fix(docs): Complete server function mutation example and fix broken links.

[mpiroc]

Fixes the following in the "learn the basics" doc for Tanstack Start:

1. The example snippet for server function mutations is incomplete and does not do what the description says it does.
2. The link to the full mutation docs was a broken, non-relative link.
3. Another link in this doc was also broken and non-relative.

I'm just getting started with Tanstack Start, so my example might not contain best practices--I would like feedback before merging!

Context (from discord):

Hi, I'm looking at https://tanstack.com/start/latest/docs/framework/react/learn-the-basics, and the example in the "Mutations" section seems to be missing a lot:

• Missing import: z
• Missing definition/import: db
• Missing definition/import: users

It also doesn't seem to do what the description says:

Here's a quick example of how you can use server functions to perform a mutation on the server and invalidate the data on the client:
Unless I'm missing something, the example doesn't do any type of invalidation.

For context, here is the example:

import { createServerFn } from '@tanstack/react-start'

const UserSchema = z.object({
  id: z.string(),
  name: z.string(),
})

const updateUser = createServerFn({ method: 'POST' })
  .validator(UserSchema)
  .handler(async ({ data }) => {
    return db
      .update(users)
      .set({ name: data.name })
      .where(eq(users.id, data.id))
  })

// Somewhere else in your application
await updateUser({ data: { id: '1', name: 'John' } })

[nx-cloud]

View your CI Pipeline Execution ↗ for commit 8a9ad9a.

Command	Status	Duration	Result
nx run-many --target=build --exclude=examples/*...	✅ Succeeded	4s	View ↗
nx affected --targets=test:eslint,test:unit,tes...	✅ Succeeded	<1s	View ↗

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☁️ Nx Cloud last updated this comment at 2025-04-16 21:41:38 UTC

[SeanCassiere]

I think there may have been some miscommunication here. By absolute, I meant what'd be used by the website.
Keep the link as it was, since the /$version/docs/ is needed.