Feature/minor improvements

[Edwardvaneechoud]

This pull request updates the documentation and improves error handling in the pyfloe library. The main focus is on standardizing import usage in all documentation examples and adding validation to catch missing columns in group-by and aggregation operations.

Documentation improvements:

• Standardized all code examples in the documentation (README.md, docs/index.md, docs/expressions.md, docs/datetime.md, docs/io.md, docs/operations.md, docs/streaming.md) to use import pyfloe as pf and reference functions via the pf. namespace, instead of individual imports. This makes examples more consistent and easier to follow. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

Core library enhancements:

• Added validation in LazyFrame.group_by() to raise a ValueError if any specified group-by columns are missing from the schema, providing clear feedback about available columns.
• Added validation in LazyFrame.agg() to check that all referenced columns in aggregation expressions exist in the schema, raising a ValueError if not.
• Updated documentation strings in LazyFrame.agg() to use the new import style for consistency.

Other changes:

• Bumped the package version to 0.1.2 in pyproject.toml.
• Minor internal import fix in src/pyfloe/core.py.