Add whatever extension is needed for :cite: support

[Tim 'mithro' Ansell]

The page at https://symbiflow.readthedocs.io/en/latest/vtr-verilog-to-routing/doc/src/vpr/index.html has

 :cite:`betz_vpr,luu_vpr_5`

On the page. This is most certainly because we are missing the correct Sphinx extension which enables citation support.