Add versioned docs support via separate gh-pages repo

[sibocw]

Background

The current docs publishing workflow (scripts/dev/push_doc_site.sh) builds the site with properdocs build and force-pushes it as a single-commit orphan to the gh-pages branch. This works well and keeps the repo lean, but it only supports one version at a time.

Goal

Support multiple doc versions (e.g. v2.0, v2.1) with a version-switcher in the Material theme, without bloating the main flygym repo with large binary assets (STL meshes, MuJoCo WASM, Three.js).

Chosen approach: separate GitHub repository for gh-pages

Create a dedicated repo (e.g. NeLy-EPFL/flygym-docs) to serve GitHub Pages. Key points:

• The main flygym repo is never touched by doc deployments — clone size stays lean
• Use mike for versioned deploys: mike deploy --push --update-aliases <version> latest
• mike incrementally commits per-version subdirectories to the docs repo's gh-pages branch; history accumulates there but it's isolated
• The Material theme version-switcher reads a versions.json that mike maintains automatically
• The manual deploy workflow (push_doc_site.sh) stays in control — mike is just a CLI call that replaces the current git-init/force-push block

What needs to change

1. Create NeLy-EPFL/flygym-docs (or similar), enable GitHub Pages on it
2. Move the CNAME (neuromechfly.org) to the new repo — DNS is unchanged
3. In push_doc_site.sh, replace the manual git-init/force-push block (~10 lines) with mike deploy --push [--update-aliases <version> latest] <version>, pointing at the new repo's remote
4. Add a few lines to properdocs.yml to enable the Material version-switcher (extra.version.provider: mike)
5. Add mike to dev dependencies

Notes

• The version is read automatically from pyproject.toml. The script then prompts:
  1. Confirm or override the version label (allows entering e.g. 2.1.1 (dev) freely)
  2. Whether to update the latest alias (answer n for dev/pre-release deploys)
• WASM mesh assets (wasm/viewer/assets/, wasm/game/assets/) only need to be regenerated when the fly model or viewer/game config changes — not on every deploy. The script already handles this correctly.
• The manual pre-deploy checks (notebook state, local preview) are fully preserved.
• The first deploy needs a clean start on the new repo (no migration of the existing unversioned gh-pages content needed).