Move the documentation to https://sky130-pdk.rtfd.io and setup redirects from https://skywater-pdk.rtfd.io

[mithro]

Related to #389 -- The location of the PDK documentation should be updated.

So that we don't break everyone's links -- the https://skywater-pdk.rtfd.io URLs should redirect to the equivalent page at https://sky130pdk.rtfd.io using the ReadTheDocs redirect support documented at https://docs.readthedocs.io/en/stable/user-defined-redirects.html

[mithro]

https://sky130.rtfd.io should also redirect to https://sky130-pdk.rtfd.io for those who forget to add the -pdk bit.