Housekeeping: rename `Untitled.ipynb`, `pc_list.ipynb`, move the Decorators folder into `notebooks/`

[utk2103]

Three inconsistencies in how the notebooks are organised:

1. notebooks/Untitled.ipynb — 15 real cells teaching conditionals, input(), and odd/even checks, but the default Jupyter filename means nobody will ever open it. Either rename it to what it teaches, or fold its cells into notebooks/Conditionals and booleans.ipynb and delete it. It also contains stray output (su-mit) from someone's local session.

2. Decorators & Namespaces/ — a top-level folder holding exactly one notebook, while every other notebook lives in notebooks/. Move it to notebooks/Decorators and Namespaces.ipynb.

3. notebooks/pc_list.ipynb — pc_list doesn't say "lists". Rename to Lists.ipynb.

Acceptance criteria

• Renames done with git mv so history is preserved.
• Spaces and & dropped from new filenames — they need URL-encoding in every link.
• Every link in README.md updated to the new paths and verified to resolve.
• The empty Decorators & Namespaces/ folder is gone.
• No notebook content rewritten in this PR — moves and renames only (except folding in Untitled.ipynb, if you choose that route).

Good first issue, no Python needed — but check README.md carefully, it links every notebook.