feat: INSTALL.md is the English installation tutorial; new tutorial docs/tutorials/first-source-to-first-answer.md with a shipped sample source; README.md links it as its first-ingestion step (#191)

Files changed:
- CHANGES.md
- INSTALL.md
- README.md
- VERSION
- docs/reference/commands/types-instructions-and-docs.md
- docs/tutorials/README.md
- docs/tutorials/first-source-to-first-answer.md
- docs/tutorials/sample-source.md
- instructions/dev/doc-pull-through.md
- tools/CONTRACT.md
- tools/chemenu/commands/docs_verify.py
- tools/chemenu/install_doc.py
- tools/chemenu/tests/test_install_doc.py

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

docs/tutorials/README.md

@@ -4,5 +4,10 @@ Learning-oriented pages: a guided first walk through an instance, start to finis
 who has not used one before. A tutorial shows one path that works; it does not list options or
 explain why.
 
-Nothing is here yet. The installation tutorial is still [INSTALL.md](../../INSTALL.md) at the
-repository root, which stays there; a walk from the first source to the first answer is planned.
+Take them in this order:
+
+1. [Install a Chemenu instance](../../INSTALL.md) - from an empty folder to a running instance.
+   It stays at the repository root, where someone who has nothing else yet finds it first.
+2. [From your first source to your first answer](first-source-to-first-answer.md) - ingest a
+   sample document, look at the pages it became, and ask the wiki a question about it.
+   [sample-source.md](sample-source.md) is the document it uses.