docs: Adding placeholder pages for various tools.

 * Analog tooling
   - Generators BAG, FASoC, etc
   - Manual tools KLayout, Magic, Innovus, etc

 * Digital tooling
   - Cadence Innovus
   - OpenROAD

 * Simulation tooling
   - ngspice
   - Cadence Spectre

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>