Redundant product name in <title> on get-started pages (seotitle suffix + siteTitle)

## Summary

The rendered `` on get-started pages repeats the product name. The
`seotitle` convention appends `| Get started with <product>`, and the title
partial (`layouts/partials/header/title.html`) separately appends
`| <product> Documentation`. The two combine to name the product twice.

## Example

`content/influxdb3/core/get-started/setup.md` has:

```yaml
seotitle: Set up InfluxDB | Get started with {{% product-name %}}
```

`layouts/partials/header/title.html` renders:

```html
<title>{{ $pageTitle }}{{ cond (ne (len $pageTitle) 0) " | " "" }}{{ $siteTitle }}
```

where `$siteTitle` is `InfluxDB 3 Core Documentation`. Result:

```
Set up InfluxDB | Get started with InfluxDB 3 Core | InfluxDB 3 Core Documentation
```

## Scope

This is sitewide, not specific to one product. The ` | Get started with
` seotitle pattern is used across get-started pages for every product
(Core, Enterprise, Cloud Dedicated, Cloud Serverless, Clustered, v2, Cloud
TSM), and the title partial always appends `| Documentation`, so the
doubling appears everywhere the pattern is used.

Evidence (not an exhaustive list):

- `content/influxdb3/core/get-started/{setup,write,query,process}.md`
- `content/influxdb3/enterprise/get-started/{setup,write,query}.md`
- `content/influxdb3/cloud-dedicated/get-started/{setup,write,query}.md`
- `content/influxdb3/cloud-serverless/get-started/{setup,write,query}.md`

## Suggested fix

Handle it in the template rather than editing every page:

- In `layouts/partials/header/title.html`, omit the page-title section suffix
when it already contains the product/site name, **or**
- Drop `| Get started with ` from the seotitle convention and let the
appended `| Documentation` carry the product name.

Either keeps a single product mention in `` and avoids per-page churn.

Start in layouts/partials/header/title.html and compare its title assembly with the seotitle examples in content/influxdb3/core/get-started/{setup,write,query,process}.md and other listed product pages. Choose the template-level handling described in the issue, then verify that rendered get-started titles contain the product name only once across the affected products.

Written by the indexing model from the issue text.