Add bigcode-tools to libraries page

[danhper]

I created a bunch of tools to work with "big code" as part of my current research and just open sourced them.
Everything quite well documented, but the toolset mostly allows to

1. Easily search and download source code
2. Parse source code to AST (Java, JavaScript and Python for now)
3. Generate nodes vector embeddings from ASTs

I wrote a short tutorial going step by step through most of the functionality.

[vraychev]

Thanks for the update.