Update gems and add incremental regeneration

The primary change in this release is an update to all outdated gems #2209, including jekyll and jekyll-archives, so that the site now supports incremental regeneration.

Additionally, we:

• Addressed bugs and accessibility concerns pertaining to the arrow icons in the site #2222
• Deprecated author frontmatter fields #2218
• Made a Github link in our footer more understandable #2224
• Added new utility classes for images and their associated templates #2225