[jrhemstad]

This PR updates the docs build/deploy flow so Sphinx emits versioned output and the version switcher resolves correctly on forks and GitHub Pages.

As a result, the docs built from main will be published under https://nvidia.github.io/cccl/unstable and future builds from release branches will be under https://nvidia.github.io/cccl/X.Y.

Key changes

• docs/gen_docs.bash now reorganizes the Sphinx build into a versioned directory, and writes root assets (index.html redirect, nv-versions.json, versions.json, .nojekyll, and objects.inv) driven by SPHINX_CCCL_VER, CCCL_DOCS_BASE_URL, and CCCL_DOCS_IS_LATEST.
• docs/conf.py derives the release version from SPHINX_CCCL_VER (fallbacks to VERSION.md/unstable) and configures the theme switcher to fetch nv-versions.json from html_baseurl, which is set via CCCL_DOCS_BASE_URL.
• docs-deploy.yml computes the base URL from the repo owner/name plus the Pages prefix, derives the deployment destination and latest flag, passes those into the docs build, and deploys under deploy/<publish_prefix> while preserving existing versions with keep_files + force_orphan.

Example versioned docs on my fork: https://jrhemstad.github.io/cccl/unstable/

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[jrhemstad]

@alliepiper would you mind giving this another review? It's changed quite a bit since your last review.

[alliepiper]

This'll be a nice improvement!