Document and type utils/iterables.py (ManimCommunity#2649)

* 🎨 Reorder functions alphabetically

Makes it easier to reference documentation alongside source code.

* 🎨 Rename concatenate_lists to flatten_lists

This is more consistent with terminology online in terms of data science.
We're flattening a 2D list into a regular one.

* 🏷️ Add types to iterables.py

Some functions require just Iterables, others Collections (sized iterables), etc.

* 💡 Add docstrings and examples to 1st half of iterables.py

Improved the clarity of some existing explanations as well.
flatten_lists was reverted to concatenate lists on closer inspection.

* 🎨 Adjust docstring format to numpy style

Use Examples header and indent docstrings description consistent with
numpy style guide as well as Manim wiki guidance.

* 💡 Add docstrings and examples to 2nd half of iterables.py

Improved the clarity of some existing explanations as well and updated
formatting to match updated style.

* 💄 Sort __all__ in file head alphabetically

Matches order of functions in file.

Co-authored-by: Darylgolden <[email protected]>

Author: tameTNT