Torizon OS docs

Deployment of the unified Torizon OS layer documentation site. The documentation site is rendered in: https://torizon.github.io/

The production design lives across three repos:

meta-torizon — the distro layer
meta-torizon-bsp — the BSP adaptations
a dedicated docs repo (this one, in production torizon-docs) that owns the shared prose and aggregates each layer's own docs into one site.

Feature under test: sync with meta-torizon-bsp/docs

Currently the per-vendor Common Torizon guides under docs/common-torizon/ are a committed snapshot so the site builds and deploys without depending on the layer repos.
In the future, those guides stay canonical in meta-torizon-bsp/docs/ and are pulled in at build time (git submodules + scripts/sync-external-docs.sh), so nothing is duplicated in version control. The sync script is kept here for regenerating the snapshot locally.

python3 -m venv .venv && . .venv/bin/activate
pip install -r requirements.txt
mkdocs serve            # http://127.0.0.1:8000

To regenerate the per-vendor snapshot from sibling clones of the layer repos:

./scripts/sync-external-docs.sh   # reads ../meta-torizon-bsp/docs, etc.

mkdocs build --strict is what CI runs; it fails on any broken link or missing nav target.