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/sample-source.md

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+# Home lab backups - how they work now
+
+Notes written after reworking the backups in September 2026. This file is the sample source for
+the tutorial "From your first source to your first answer"; the machine and its setup are made
+up for it.
+
+## The machine
+
+**Atlas** is the small file server in the hall cupboard: a fanless mini PC with two 4 TB disks
+in a mirror, running Debian 13. It holds the family photo archive, scanned documents and the
+configuration of every other machine in the house. Everything else on the network is replaceable;
+what is on Atlas is not.
+
+## The tool
+
+Backups are made with **restic**, an open-source backup program. restic splits files into chunks,
+stores each chunk only once and encrypts everything before it leaves the machine, so the backup
+target never sees a file name or a byte of content in the clear. A restic *repository* is the
+storage location it writes to; a *snapshot* is one backup run.
+
+restic was chosen over plain `rsync` for two reasons: deduplication keeps the daily runs small,
+and encryption makes it safe to put a copy on storage someone else operates.
+
+## How it is set up
+
+Atlas runs restic every night at 02:30 from a systemd timer. Each run writes one snapshot to two
+repositories:
+
+- a USB disk that stays plugged into Atlas, for fast restores;
+- a rented storage box at a hosting provider, reached over SFTP, as the copy outside the house.
+
+Snapshots are thinned out by `restic forget` after each run: the last 7 daily, 5 weekly and
+12 monthly snapshots are kept.
+
+## Why two targets
+
+The setup follows the **3-2-1 backup rule**: keep three copies of the data, on two different
+kinds of storage, with one copy off site. Here the three copies are the mirror on Atlas itself,
+the USB disk, and the storage box. A fire or a burglary takes the first two; the storage box is
+what is left.
+
+The mirror alone does not count as a backup. It protects against a dead disk, but a deleted or
+encrypted file is deleted or encrypted on both disks at once.
+
+## What is still open
+
+The restore has been tried once, for a single folder, from the USB disk. It worked and took
+under a minute. A full restore from the storage box has not been tried.