Rebuild homepage: leaner intro + Variant 1 "In this documentation"

[rkratky]

What this PR does

Rebuilds the docs homepage (docs/index.md) with two changes:

1. Leaner introductory paragraphs

The four leading paragraphs under the H1 are condensed to two while preserving all information:

• What Ubuntu is + what toolchains are available (Python, Go, Rust, GCC, Clang, .NET, Java, Zig)
• What the docs cover + audience + scope note ("does not teach programming", "no prior Ubuntu experience required")

2. Variant 1 — "In this documentation" organized by lifecycle stage

The "In this documentation" section is restructured into two subsections:

• Introduction — platform-level setup and Ubuntu-specific concepts (install Ubuntu, Git, IDEs, packaging, toolchain availability)
• Language toolchains and support — one row per language, each linking the install guide, first-program tutorial, and version availability reference

This variant groups content by where it falls in the development workflow, making it easy to find pages by lifecycle stage.

Unchanged sections

The toctree, "How this documentation is organized", and "Project and community" sections are unchanged.