Docs/http error pages

[panch1739]

This PR adds the docs for the HTTP error page pattern.

Please make sure your PR follows the contribution guidelines
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Documentation.
Examples.
Dashboards Demo.
Playwright report.

Coverage Reports:

• charts-ng
• dashboards-ng
• element-ng
• element-translate-ng
• maps-ng
• native-charts-ng

[gemini-code-assist]

Code Review

This pull request introduces a new documentation page for HTTP error pages, detailing their usage, design anatomy, and visual guidance, and adds it to the navigation menu in mkdocs.yml. The review feedback suggests correcting the capitalization of the navigation entry to comply with the repository style guide, as well as fixing minor grammatical issues in the documentation text, such as adding a missing article and correcting pronoun agreement.

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[kfenner]

@panch1739 Great work, looks perfect to me, small and concise.

@mistrykaran91 Do you happen to have time to create the code example(s) for each of the 6 cases described in the Figma and add it to this PR here? https://www.figma.com/design/wCNnN0JDC74XrZyY6EVrcP/%F0%9F%94%B5-HTTP-error-pages?node-id=5112-2460&p=f&t=U6FdoAPJ19ytCbHw-0

We don't want a separate component implemented. We should just have example(s) using the si-info-page component.

I'm not sure whether we should have only one example with a dropdown to switch between the error codes or actually 6 separate examples. I would say a single one is nicer and easier to handle. WDYT?

/cc @spike-rabbit @timowolf