[eigen_I] Improvement suggestions

This PR collects feedback from the reading group on eigen_I.

Content

• Add an introduction to what eigenvalues and eigenvectors are. We can give the formula at the beginning and a visualization showing that eigenvectors are invariant in the transformation and eigenvalues are the scaling factors. We can link this further to the vector field plots in the exercises.
• Give a better motivation for studying eigenvalues and eigenvectors: (1) mention that it is used in Markov Chain and more advanced lectures in intermediate series; (2) add links to the list of areas that use eigenvalues below: "At the same time, these concepts are extremely useful for".
• Transformation plots (in https://intro.quantecon.org/eigen_I.html#square-matrices) should be hidden and potentially relocated, and we should explain the details of the transformation (@jstac).
• Hide functions used to generate the visualizations and reference the function name in the main text.
• Reformat the subtitles to meet the style sheet (only the first word and proper names should be capitalized).
• We can give an overview of what is the Neumann Series Lemma at the beginning of the section.

Code

• Some labels in the graphs are not in the math mode $ $.
• In this plot, the label for the transformed plot should be $Ax_1, Ax_2, ..., Ax_k$ instead of $Bx_1, Bx_2, ..., Bx_k$, and the second label should be $BAx_1, BAx_2, ..., BAx_k$
• We can remove the default values in

def grid_composition_transform(A=np.array([[1, -1], [1, 1]]),
                               B=np.array([[1, -1], [1, 1]])):

.

• Rewrite the code in exercise 7.1 to using while loop and error tolerance.

Start by reading the eigen_I lecture at intro.quantecon.org/eigen_I.html, including the square-matrices, rotate-then-shear, Neumann Series Lemma, and exercises sections. Inspect grid_composition_transform and exercise 7.1 in the notebook, then work through the checklist. Done means the requested content, labels, formatting, hidden visualization helpers, defaults, and loop changes are addressed.