Add extensions and minor adjustments

[EwanLyon]

Added a dedicated extensions page and made some more minor tweaks/QoL improvements.

Added:

• Added Extensions document
  • Which type of export to use in the index.ts code examples. I don't mind both but it can be confusing for new people.
• New table styling

Adjusted:

• Update dependencies to latest
• Remove "using npm" from sidebar
  • It's just a guide on how to install a package using npm and I don't think that's relevant for NodeCG docs
• Convert commented file names/paths to code block titles. Affected files:
  • custom-routes.md
  • nodecg-configuration.md
  • nodecg.md
  • sendMessage.md
• Remove arrows from NodeCG goals
• Added descriptions when they were missing. Affected files:
  • findCue.md
  • playSound.md
  • stopSound.md
  • waitForReplicants.md
• Rename CURR_BUNDL to be "Current Bundle". Affected files:
  • getDialog.md
  • getDialogDocument.md
  • listenFor.md
  • readReplicant.md
  • unlisten.md
• Remove parenthesis from titles to be consistent. Affected files:
  • getSocketIOServer.md
  • log.md
  • mount.md
  • stopAllSounds.md

Fixed:

• Light mode mobile sidebar text colour
• Light mode breadcrumb text colour

Closes #3