docs(site): redesign Community Renderers directory for scannability and platform filtering #4232

Context

On the Community Projects page (docs/site/src/content/docs/ecosystem/renderers.mdx), the Community renderers section is a single flat 5-column Markdown table with 20+ entries across web, mobile, desktop, cross-platform, and terminal ecosystems, with additional community submissions in flight.

As raised by @ditman, the flat table gets harder to scan as new rows are added, and readers have no quick way to narrow the list to their target platform or framework (for example, viewing only React or mobile renderers).

Proposed changes

• Group or filter by target platform / framework: Organize community renderers into tabs (such as Starlight <Tabs> / <TabItem>) or add platform/framework filtering (e.g., React & Web, Mobile & Native, Cross-Platform, Terminal / CLI) so developers can jump straight to renderers for their stack.
• Shorten descriptions and tighten layout: Trim verbose descriptions and refine the table or card layout within each group so entries stay easy to scan as the list grows.
• Update contribution instructions: Adjust the How to submit a project section in docs/site/src/content/docs/ecosystem/renderers.mdx to match the updated grouping structure and concise description length.