Add links to the points in the "Why SymbiFlow?" section

[rw1nkler]

This links should be added to the following points:

• Free & Open Source
• Multi-platform
• Vendor-neutral
• Interchangeable tool suite
• Growing community
• Look inside and improve

[rw1nkler]

@mithro do you have any ideas about those links?

Maybe we can link:

• Growing community -> IRC / Slack
• Look inside and improve -> GitHub
• Interchangeable tool suite -> Diagram from docs
• Vendor-neutral -> link to the section with Supported Architectures

But what about the others?