Add an end-user Overview of all add-ons + credit upstream fork

[timlinux]

User story

As a new timvim user, I want a single Overview page that shows every
add-on in the distribution and what it is for, so that I can understand what the
editor gives me and jump to the right guide — without reading the Nix config.

As a maintainer, I want that Overview generated from a source-of-truth
manifest and drift-checked against the live config/ tree, so it can never
silently fall out of date when we add or remove a plugin.

Context

flowchart LR
    cfg["config/*.nix<br/>(live add-ons)"] -->|drift-check| gen["lib/gen-addons-docs.py"]
    man["lib/addons.json<br/>(source of truth)"] --> gen
    gen --> svg["addons.svg<br/>(diagram)"]
    gen --> tbl["overview/addons.md<br/>(linked table)"]
    svg --> book["Handbook · Overview"]
    tbl --> book
    man -. "add a plugin without<br/>documenting it" .-> fail["build fails"]

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Success criteria

• New Overview section in the handbook, between Getting Started and the
  User Guide, with an end-user diagram + a fully linked add-ons table.
• Diagram is a Python-generated, brand-coloured SVG (kartozaColors).
• Table + diagram render from lib/addons.json; adding/removing a plugin
  without updating the manifest fails nix run .#handbook-addons.
• Generator wired into the docs + release CI and the PDF chapter order.
• Upstream Schrovimger (jack-thesparrow) fork and NVF credited in the
  README and the handbook.

Job size

M (~1 day).

Epic

Documentation handbook.

Tags

documentation, enhancement, handbook