Added a new padding function to transforms.py, called it in backend_bases, and updated documentation where pad_inches shows up

PR summary

Solution for #11764, allowing pad_inches to be set for each border when saving a figure.

• Why is this change necessary?
  Difficult to get compact but suitably bounding borders in savefig with extra artists in some cases
• What problem does it solve?
  E.g. a plot with broken axes made with subfigures and diagonal lines indicating the break is hard to bound correctly. This adds a crude but simple way to modify the bounding box as needed.
• What is the reasoning for this implementation?
  Seems like the simplest way to do it.

import matplotlib.pyplot as plt
import numpy as np

plt.plot(np.arange(1, 10))
plt.xlabel('Test')
plt.ylabel('Test2')
plt.title('Typical Title')
plt.savefig('test_float_0.png', bbox_inches = 'tight', pad_inches = 0)
plt.savefig('test_tuple_0.png', bbox_inches = 'tight', pad_inches = [0, 0, 0, 0])

plt.savefig('test_float_1.png', bbox_inches = 'tight', pad_inches = 1)
plt.savefig('test_tuple_1.png', bbox_inches = 'tight', pad_inches = [1, 1, 1, 1])
plt.savefig('test_tuple_mixed.png', bbox_inches = 'tight', pad_inches = [0, 2, -0.2, 4])

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• [?] Documentation complies with general and docstring guidelines