docs: fixed md formatting, misspellings and broken links

[czar0]

Description

As an eager student (and great fan! 😄) of this fantastic course, I wanted to provide a small contribution to it by fixing some minor "bugs" I encountered here and there while going through those pages.
Most of the corrections have been done using automated tools (see list below) and then checked manually.

Changes

• Fixed all markdown formatting
• Corrected misspellings in en-US locale
• Fixed broken links and replaced missing resources (e.g. deleted tweets) with Internet Archive page when available
• Replaced all HTML code with equivalent markdown syntax
• Escaped _ character in all {target=_blank} to avoid conflicts with italicize markdown syntax
• Added {target=\_blank} to all hyperlinks
• Added some TODOs for easier issue tracking

Tooling

• Markdown syntax corrections based on markdownlint's VSCode extension rules
• Markdown formatting using commonmark (integrated into prettier code formatted)
• Markdown syntax highlighting based on linguist (supported by GitHub's processor)
• Spell check using Code Spell Checker for en-US