[README] update Usage section

[Zelig880]

Update the usage section to be easier for people to find things and also for people to add it

AC:

• Add a little sentence at the start of the Update section to explain that the section will include quick links to specific code implementation that can be used for learning purposes or to be reused in other projects to get started with Solana Web3.
• Remove all the code currently shown and replace it with links that highlight specific lines (you can do this adding L1-L10 to highlight the first 10 line for example https://github.com/thisdot/blockchain-prediction-game/blob/main/api/feed/getLatestDataRound.js#L1-L10)
• Provide some context on each of the section for people to understand what is the section about. For example in chainlink explain what chainlink is used for and provide some links.
• Go through all the points in this section (Usage) and make sure we explain Why we do something and not just WHAT. So for example the "How to setup anchor", we should tell people what is anchor and why we use it, and then tell them how to use it. This definition should be link heavy (so get definition of what and why from the main sites and link them to external documentations).