[Docs] Remove legacy mobx example (#40304)
## Info
This PR merges with-mobx-lite into with-mobx. After that we will migrate `with-mobx` to typescript.
## Context
As discussed in https://github.com/vercel/next.js/pull/40302#issuecomment-1239238966 we are going to merge the mobx examples.
#40302
## Documentation / Examples
- [x] Make sure the linting passes by running `pnpm lint`
- [x] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
