docs: add Topo documentation homepage

[ZachMaso]

Summary

• Add a dedicated Topo documentation landing page
• Explain the Topo workflow with concise overview, guide, and command sections
• Reuse the overview diagram as a shared static asset

Why

Give Topo users a clear entry point that explains what Topo does, why to use it, and where to begin.

Full styling relies on merging of https://github.com/Arm-Debug/arm-docs-github-action/pull/79 but UI is visible and functional without this PR merged.

[ZachMaso]

Screen.Recording.2026-07-21.at.15.42.14.mov

[awphi]

Nice - structure and styling look nice, excited to see it with the v5 styling merged! Handful of comments focused on minimizing unused stuff.

One other bit; we can delete docs/README.md as that was just a placeholder landing page.

[ZachMaso]

Review comments addressed:

• Added a Configure step using topo clone, so the workflow now matches the diagram.
• Removed the SHELL labels.
• Made the command blocks line up across the cards.
• Removed the unused tutorial card handling.
• Simplified the links so they all use the Docusaurus Link component.
• Moved the external-link icon into its own SVG file.
• Imported the diagram directly instead of loading it as an image.
• Removed the unused disabled and external card options.
• Made card links required and removed the fallback link to a page that doesn’t exist.
• Removed the unused button, install, code block, card footer, dot and tutorial styles.
• Changed the overview diagram path to /img/topo-overview.svg.
• Removed docs/README.md, as it was only a placeholder.

Other changes made:

• Changed the mobile layout so the title and description appear before the diagram.
• Limited the workflow cards to two columns, dropping to one column on smaller screens. This avoids a row containing a single card.
• Made the workflow tag backgrounds match the code sections in both themes.
• Made the workflow tag borders match the panel outlines in dark mode.
• Fixed the clickable panels so their hover border is visible in dark mode.
• Made the neutral diagram labels, arrows and lines black in light mode and white in dark mode.
• Kept the purple and green parts of the diagram unchanged.

[awphi]

This looks really good especially with #378 merged in for the new global styling!