docs: restructure and improve documentation website

[wgtmac]

Changes

README.md

• Add License badge
• Split Requirements into Required and Optional, add Ninja
• Add Quick Start section (clone → build → test in one go)
• Add CMake Build Options table with all available options
• Add "Build with Meson" section with Meson-specific options
• Move "Customizing Dependency URLs" after Build sections
• Add ctest -R example for running specific tests
• Normalize cd paths (assume user is already in project root)

Website restructure (mkdocs/)

• Navigation: Home → Getting Started → Contributing → Releases (3 sub-pages) → API Documentation
• New index.md: Project introduction, key features, quick links, and community info (was previously the Contributing page)
• New getting-started.md: Build/install instructions extracted from Contributing, with CMake/Meson options tables and dependency URL customization
• New contributing.md: Coding standards (naming convention table), development environment setup, submission workflow, and community links
• New release-process.md: Release manager guide extracted from dev/release/README.md
• New verify-rc.md: RC verification guide with both automated script and manual steps
• Rewritten releases.md: Summary table + per-version highlights; 0.2.0 content aligned with official blog post; corrected release dates (0.1.0: Sep 10 2025, 0.2.0: Jan 26 2026); removed inaccurate pre-built binaries and package manager claims

Fixes

• Rename docs/assert/ → docs/assets/ (typo)
• Remove Apache license header from CNAME file (may break GitHub Pages parsing)
• Add site_description to mkdocs.yml for SEO
• Enable navigation.sections and toc.integrate for cleaner layout

Screenshots

[wgtmac]

I've polished the website a little bit :) @Fokko @kevinjqliu

[kevinjqliu]

Looks great, ty!

[wgtmac]

Thank you @kevinjqliu! Let me merge it to refresh the site!