Set up readthedocs pull request rendering

[mithro]

Expected Behavior

Sending a pull request on this repository shows how it will affect the documentation around the standard cells found at https://gf180mcu-pdk.rtfd.io

This could be done by setting up a conf.py and Makefile under the docs directory with similar config as the gf180mcu-pdk repository but just including the standard cells library section of the documentation.

Another option would be to automatically send a pull request to gf180mcu-pdk repository with the submodule updated and copying the details into the pull request here.

Actual Behavior

A change has to be merged and then a dependabot pull request updates the gf180mcu-pdk repository which shows the changes.

[mithro]

@proppy - I believe this has been done?