bring back some of the styling from the previous docs website

[StephenFluin]

• for e.g. font style,
• links
• crowded copy button,
• mainnet/testnet switching,
• syntax highlighting,
• underline below the sections looks weird,
• notes/warning symbols are not as prominent anymore).
• Resources should have a button in the nav bar too (seems out of place compared to developers/node operators etc.)
• the fully expanded table of contents on the left feels like an info dump

CC @milapsheth

[StephenFluin]

Take a look at this feedback @ffe9f8 and let's decide if we want to make more changes.

[ffe9f8]

Some additional thoughts brought up in the #developer-experience-discussions Slack channel:

• Make layout and visuals look more like GitBook's to optimize for user familiarity
• Wider margins for main body (similar to Notion)
• More vertical spacing
• Cleaner code formatting
• More consistent with main text
• Familiar syntax highlighting theme