[data grid][docs] Fix Localization page title

[oliviertassinari]

Match with the sidenav:

No reasons for the two to be different.

Also, closer to the baseline model we aim to copy in X: https://mui.com/material-ui/guides/localization/

---

Off-topic. On the localization topic, more generally, https://mui.com/x/react-date-pickers/localization/ went in a different direction, we will likely need to cherry-pick the best of the two worlds to unify the approach so it's simple to follow for users. https://mui-org.slack.com/archives/C074TBYQK2T/p1745238404469119 is a great signal that the docs structure for localization can be fixed.

But at least, in the case of the pickers, the page was split in different sub-pages, so on that very topic that this PR touches on, it also fit well in the global story.

A proposal:

• localeTextConstants.ts -> enUS.ts. Pickers seem better
• Add English (United States) / en-US in the list of supported locales
• Have this structure:

# Localization

> Localization (also referred to as "l10n") is the process of adapting a product or content to a specific locale or market.

The default locale is English (United States). If you want to use other locales, follow the instructions below.

## Localize text

### Translation keys

### Set translations locally

### Set translations globally

### Example

-### Supported locales
+### Built-in locales

<!--
Supported locales is confusing, it implies that what's not in there is not supported.
No, it's only not included in the default build.
-->

-## RTL Support
+## Right-to-left support

<!-- Consistent header -->

## Localize date format

## UTC and timezones (if relevant)

## Calendar systems (if relevant)

and break it in different pages if it's too heavy for one.

[github-actions]

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[mui-bot]

Deploy preview: https://deploy-preview-17475--material-ui-x.netlify.app/

Updated pages:

• docs/data/data-grid/localization/localization.md

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