Prune template content and simplify README (remove example code, containers, and docs)

[ttuff]

Motivation

• Reduce the repository from the full starter/template kit down to a minimal project scaffold and README.
• Remove heavyweight example assets, container configs, and exploratory code that are not needed for the pared-down repo.
• Provide a concise README and local preview instructions for the minimal MkDocs site.

Description

• Deleted many template and example files including CITATION.cff, LICENSE, TEMPLATE_GUIDE.md, TEMPLATE_SETUP.md, and multiple docs and code examples under code/ (scripts, notebooks, and markdown docs) to remove the original template content.
• Removed container and CI-related artifacts under containers/jupyterlab/ such as Dockerfile, environment.yml, and jupyter_notebook_config.json to drop the custom local environment setup.
• Removed site configuration and assets including docs/_config.yml and a large PDF asset under docs/assets/ to simplify the published site.
• Rewrote README.md to a short, focused file named basic_OASIS content that lists the repository essentials and includes a minimal local preview command sequence using pip install mkdocs mkdocs-material and mkdocs serve.

Testing

• No automated tests or CI jobs were executed as part of this change.
• Manual inspection of the tree was used to confirm the deletions and that the new README.md contains the minimal instructions; there were no build/test artifacts to run.

---

Codex Task