fix: README link fixes & formatting

[igneel64]

Description

• Review broken links
• Autohighlight/Format code examples using the appropriate tag.
• Blank target for third-party links

[nachoiacovino]

Doesnt' look like target="_blank" works on GitHub. I tried the commit here https://github.com/clerkinc/clerk-sdk-node/tree/6ddace8c502524e04d8bcb010cea1d8424780796 and they don't open on a new tab.

More info: https://stackoverflow.com/questions/41915571/open-link-in-new-tab-with-github-markdown-using-target-blank

We should try and see if there's another way to do it and if there isn't, we should revert those changes to make everything looks cleaner.

[yourtallness]

#lgtm

[igneel64]

@nachoiacovino Thanks for your comment!

• I would keep the formatting as it is since I would not consider GH as the only place markdown is parsed
• Also if and when GH supports it, it will probably be in the format that the standard supports (which is HTML in MD).
• For the clean part, I do not have any strong feel about it, the only thing is that the full link tag is more expressive.