[Internal]: Add a made-up brand example to Kibana custom branding

### Description

**What:** Add a realistic worked example to [Customize Kibana branding](https://www.elastic.co/docs/deploy-manage/kibana-custom-branding) that uses a made-up brand (not Elastic, and not a real customer).

**When:** Follow-up after #8195 / https://github.com/elastic/docs-content/pull/8190. Do not block the availability tagging PR.

**Why:** The how-to lists settings and size limits, but it does not show a complete example. A made-up brand would help readers see how logo, organization name, page title, and favicons work together, including which assets are uploads and which are URLs.

Suggested shape (final placement can wait until drafting):

- Invent a small fictional company and keep the same name across every setting.
- Show example values that match the product constraints (logo ≤128×128 px and under 200 KB, organization name image ≤200×84 px and under 200 KB, favicon 16×16 px).
- Call out that **Page title** is stack-only.
- If a screenshot is worth adding, flag it in the PR. Do not use a real customer's branding.

### Resources

- How-to: https://www.elastic.co/docs/deploy-manage/kibana-custom-branding
- Companion reference: https://www.elastic.co/docs/reference/kibana/advanced-settings#kibana-custom-branding-settings
- Availability tagging: #8195, https://github.com/elastic/docs-content/pull/8190

### Which deployment methods does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

The example should include **Page title** for Stack only. Serverless readers should still get logo, organization name, and favicon values.

### What Elastic Stack release is this request related to?

N/A

### Serverless release

N/A

### Collaboration model

The documentation team will create the first draft

### Point of contact.

**Main contact:** @florent-leborgne

**Stakeholders:** @nastasha-solomon