Use consistent link color, refactor text content and doc card components

[damianstasik]

Closes #130

This PR does a few things:

• Makes sure all links have consistent look on hover (they should not use default theme's link color)
  • The exception to this rule are admonitions (always lightest gray in dark mode, and darkest gray in light mode)
  • And active navbar & TOC items, they just get an underline.
• Cleans up text content to use the same TextContent component, now docs, blog and manifesto share the same styles.
• Makes longer admonitions a bit more readable by increasing text contrast, the space between type and content and adding consistent line height to content.
• Fixes a few times we had links wrapping headings, the correct syntax is the other way around.
• Refactors the doc card component to use tailwind for better styling of the hover state. I just reused the styles from paginator nav links.