[Docs] Fix: Dead link to JS build setup tutorial in scripts readme + incorrect use of absolute URL's.

[dontfeedthecode]

What?

When viewing the scripts package README the link to JavaScript Build Setup tutorial within the Setup section leads to a 404. In addition, all links referencing the Gutenberg repository in this README had absolute paths including the GitHub URL.

Why?

The link to JavaScript Build Setup tutorial should lead to a valid URL. Absolute paths containing the GitHub URL contravenes the Using Links section of the contributor guidelines.

How?

There is a reference to this outdated JavaScript Build Setup tutorial URL in the WordPress Developer Resources how-to guides. A decision to redirect this link to https://developer.wordpress.org/block-editor/getting-started/fundamentals/javascript-in-the-block-editor/ had already been made in the past, I've updated the link in the README accordingly.

All links to the Gutenberg repo (including the updated link referenced above) have had their URL's updated to remove the domain, e.g. https://github.com/WordPress/gutenberg/blob/HEAD/packages/scripts/CHANGELOG.md becomes /packages/scripts/CHANGELOG.md.

Testing Instructions

The link pictured below should now say "Working with JavaScript for the Block Editor" which is the correct page title for the updated URL and no longer lead you to a 404 page.

There should be no absolute references to https://github.com/WordPress/gutenberg/blob/* or https://github.com/WordPress/gutenberg/tree/* in links within the README.