Add guidance for alert syntax usage in style guide

[harikapadia999]

Description

This PR adds guidance to the contributing style guide explaining when to use the GFM alert blockquote Markdown syntax versus the Hugo alert shortcode.

Changes

• Added new "Alert syntax" section under the Markdown section
• Recommends GFM alert blockquote syntax as the default choice for better portability and readability
• Explains when to use the Hugo shortcode (for advanced features like custom titles)
• Includes examples for both approaches

Related Issue

Fixes #8485

Checklist

• Added clear guidance on alert syntax preference
• Included examples for both GFM and shortcode syntax
• Explained when to favor each approach
• Followed existing style guide formatting

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: harikapadia999 / name: HARI KAPADIA (223d043)

[theletterf]

@open-telemetry/docs-maintainers This might be a good time to decide on the syntax we want. AFAIK, we prefer the Hugo shortcode.

[svrnm]

@open-telemetry/docs-maintainers This might be a good time to decide on the syntax we want. AFAIK, we prefer the Hugo shortcode.

See the issue for that discussion #8485, my understanding was that with the recently added support for github formatted alerts we want to prefer them over the shortcodes