DOC: Rename basics to fundamentals + added description

[cooperrc]

Rename "NumPy basics" to "NumPy Fundamentals" and reorganize content

The NumPy Basics section consists of the following content:

• Data types
• Array creation
• I/O with NumPy
• Indexing
• Broadcasting
• Byte-swapping
• Structured arrays
• Writing custom array containers
• Subclassing ndarray

These articles have very useful information for users, but they do not cover the basics. The basics are covered in Absolute Basics for beginners.

This PR adds a description to the section as explanation material and relabels the section as "NumPy Fundamentals". This documentation space can be used to clarify parts of NumPy and describe design decisions in NumPy. Most of this content is already there, but it will help to capture it in its own section and label it.

This PR reorganizes the NumPy Basics Fundamentals to the following order:

• Array creation
• Indexing
• I/O with NumPy
• Data types
• Broadcasting
• Byte-swapping
• Structured arrays
• Writing custom array containers
• Subclassing ndarray

[rgommers]

Makes sense, the "basics" for not-at-all-basic-things had bothered me for a while.

[rgommers]

Merged, thanks @cooperrc