docs: Adding default Sphinx setup.

 * Uses the following sphinx extensions;
    - [x] [sphinx-verilog-domain](https://github.com/SymbiFlow/sphinx-verilog-domain)
    - [x] [sphinxcontrib-hdl-diagrams](https://github.com/SymbiFlow/sphinxcontrib-hdl-diagrams)
    - [x] [Symbolator](https://github.com/SymbiFlow/symbolator)
    - [x] [sphinx_symbiflow_theme](https://github.com/SymbiFlow/sphinx_symbiflow_theme)

 * Adds a few custom roles useful for interlinking the PDK modules.

 * Uses `make-env` to enable a conda environment containing required
   binaries like Yosys, netlistsvg, etc.

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