Fix broken torchao documentation links (#21547)

### Problem
Several documentation pages link to torchao URLs that are dead. Two of
them are subtle: the URL returns HTTP 200, but the page is a stub whose
only content is a meta-refresh redirect back to a 404. So a status-code
check passes while a human clicking the link still lands on a 404.

### Fix
Point each link at the page that actually serves the content. The pt2e
tutorials now live under `pt2e_quantization/`, which is the path this
repo already uses in `arm-vgf-overview.md`, `nxp-quantization.md`, and
`quantization-overview.md`, so this also makes the links consistent.

| Old | New |
|---|---|
| `ao/main/tutorials_source/pt2e_quant_ptq.html` |
`ao/main/pt2e_quantization/pt2e_quant_ptq.html` |
| `ao/main/tutorials_source/pt2e_quant_qat.html` |
`ao/main/pt2e_quantization/pt2e_quant_qat.html` |
| `ao/main/serving.html#mobile-deployment-with-executorch` |
`ao/main/eager_tutorials/serving.html#mobile-deployment-with-executorch`
|
| `ao/stable/generated/torchao.quantization.quantize_.html` |
`ao/stable/api_reference/generated/torchao.quantization.quantize_.html`
|

The last one now goes to the specific `quantize_` API page rather than
the general quantization index.

### Verification
Each target was fetched and confirmed to render the expected page,
following both HTTP redirects and html meta-refresh, rather than only
checking the status code. The `#mobile-deployment-with-executorch`
anchor was confirmed present in the served page.

### Files
xnnpack, samsung, coreml and arm-ethos-u quantization docs, the LLM
export guide, and the Arm Ethos-U docgen template. 11 links total. The
docgen template and its generated page are kept in sync.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils
@Sebastian-Larsson @robell @rascani