Fix broken link for rdkit-sys in readme

[bmp192529]

The link for rdkit-sys in the rdkit readme goes to a page that doesn't exist anymore; the page was probably deleted when rdkit-sys was merged into this project. Fixing that and a few other broken links I noticed (which I can add here as comments) would make this project easier to use for newcomers.

[bmp192529]

There is also a broken link in the readme for the rdkit-sys section linking to the rdkit crates.io page, that no longer exists.