Modularize and enhance Publish/Subscribe content

Done Criteria

• Create an introductory doc for publish/subscribe, similar to the other core component sections (e.g. Transports, Steam Multiplexing). The title can be "What is Publish/Subscribe"
• Create a gossipsub doc. Content updates are captured in https://github.com/libp2p/docs/issues/169.
• Add more examples as part of "example uses of pub/sub:"
• General writing style updates.
  • Gramatical.
  • There are descriptions that are currently commented out that were used as the image descriptors - these are useful descriptions that should be incorporated into the actual text on the page.

Why Important

While a solid explainer, there is opportunity to improve the current material and modularize the doc itself.

Notes

• The section would resemble the following structure:
  • What is Publish/Subscribe
  • GossipSub
• For now, we can update the images to have a transparent background so they fill the pages better (i.e. when in dark mode). However, we should eventually update all the pub/sub diagrams as part of the unified libp2p diagrams.