[Code scan] Clean up non-portable example notebooks and unpublished pip command

This issue comes from a Codex global repository scan.

## Problem
Some examples are not reproducible from a fresh checkout.

`example/dftio_demo.ipynb` executes `Field.from_cube("xxx.cube")`, but no `xxx.cube` fixture exists in the repository:

https://github.com/deepmodeling/dftio/blob/c9d128f24a74ef2911e1a28f5640357488beb196/example/dftio_demo.ipynb#L87

The SIESTA notebooks contain private absolute paths and stale imports, so they fail outside the author's machine:

https://github.com/deepmodeling/dftio/blob/c9d128f24a74ef2911e1a28f5640357488beb196/dftio/io/siesta/try_siesta.ipynb#L11

https://github.com/deepmodeling/dftio/blob/c9d128f24a74ef2911e1a28f5640357488beb196/dftio/io/siesta/try_siesta.ipynb#L65

https://github.com/deepmodeling/dftio/blob/c9d128f24a74ef2911e1a28f5640357488beb196/dftio/io/siesta/check_siesta.ipynb#L9-L12

The README also shows an uncommented `pip install dftio` command under a “coming soon” heading, which fails while the package is not published to PyPI:

https://github.com/deepmodeling/dftio/blob/c9d128f24a74ef2911e1a28f5640357488beb196/README.md#L42-L46

## Suggested fix
Add a small cube fixture or make that notebook cell non-executed, replace private paths/stale imports in SIESTA notebooks with repo-relative examples, and remove or comment out the PyPI install command until publication.