Restructure language introduction around reader experience

[floitsch]

The language introduction mixed beginner lessons with advanced details, while the comparison page mostly translated syntax. Provide a complete primary introduction for experienced programmers and a separate, slower path for people new to programming.

• Make “Toit for programmers” the main tour, covering syntax, functions, values and types, strings, blocks and lambdas, control flow, collections, classes and interfaces, errors, imports, and tasks.
• Explain Toit directly and use C/C++, JavaScript, Python, and Java/Kotlin comparisons where useful. Readers do not need every comparison language, and essential content is not hidden behind fluency toggles.
• Give blocks substantial treatment, including captured locals, block-taking methods, result and return behavior, lifetime restrictions, and the stack-based representation.
• Keep normal examples visible and offer supplementary expandable collections for variations and edge cases.
• Split the beginner path into three lessons: a first temperature report, lists and functions, then objects. Introduce concepts when needed and include practice at each stage.
• Keep language-specific guides as optional entry points. Separate reference navigation from the learning paths and document the audience and example conventions for contributors.

Validation: 56 runnable examples pass toit run -Werror; the helper library in the two-file import example passes toit analyze -Werror. The production build and internal-link checks pass across 146 HTML files. New section links and git diff --check also pass.