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>
diff --git a/docs/sim/spectre.rst b/docs/sim/spectre.rst
new file mode 100644
index 0000000..5090786
--- /dev/null
+++ b/docs/sim/spectre.rst
@@ -0,0 +1,2 @@
+TODO: sim/spectre
+=================