Fix/pytorch accessibility

[Mwarsame871]

Accessibility Fixes — Introduction to PyTorch for HPC

Fixes accessibility issues found by Pa11y CI and the RC Learning
Accessibility Checklist for the pytorch-hpc workshop.

Changes

• search.html: uncommented label, added aria-label to search input
• docs_sidebar.html: added role=search and hidden submit button
• book_sidebar.html: added role=search and hidden submit button
• overview.md: added alt text to 4 activation function images
• simple_nn.md: added alt text to neural network diagram
• custom.scss: fixed contrast ratio on code block function/class names
• custom.scss: set body line-height to 1.6 for WCAG 1.4.12
• custom.scss: added sidebar contrast fix for light and dark mode

Testing

All 7 pytorch-hpc pages pass Pa11y locally with 0 errors.