docs: rewrite readme and examples for 1.0

[devCrossNet]

What is accomplished by your PR?

README

• New structure: Getting started (questions → navigation → usage → end of flow), Guides, API table
• New content that was missing before:
  • component types and where the next question comes from
  • options that depend on former answers (nested data shape)
  • navigation output properties (value, active, hasValue, isValid, …)
  • validation and reset of dependent answers
  • skipping questions with NO_VALUE
  • jumping to questions, restoring state
  • defaultValue is not saved automatically
• Fixed: wrong code samples (nextItemId: {}, onSubmit(value: any) in a JS block), typos, /tree/ link to a file, master link, line-number link to types.ts

Examples

• Every example README explains what it shows and links data.ts and the spec
• custom-component-types and extending-data-definition list every override and why
• New example navigation (categories with and without own question, jumping, changing answers) with 4 tests
• restore-state moved into its own folder with a README

Fix (separate commit, breaking)

hasValue of a parent navigation item with its own question was true as soon as it had a child, even without any answer. It is now true only when the parent or a child has an answer.

Other

• Fixed misleading comments about _alwaysPossibleFollowUpQuestionComponents (it checks the component type of the question itself)
• CONTRIBUTING.md: replaced the outdated next branch with main

Is there something controversial in your PR?

• The hasValue fix is a breaking change (planned for 1.0).
• The Twitter link is removed (profile no longer exists).

Checklist

• Run unit tests to ensure all existing tests are still passing
• Add new passing unit tests to cover the code introduced by your PR
• Change documentation for the code introduced by your PR

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