getting-started: add references to interesting repos in the org

[umarcor]

This PR adds references to some interesting repos in the org, which might go unnoticed:

• sphinxcontrib-hdl-diagrams
• yosys-symbiflow-plugins
• symbiflow-bitstream-viewer
• fasm
• fpga-tool-perf

[umarcor]

Moved to f4pga/f4pga-website#2.