How to: Setup your docfx site so it can be edited in GitHub Codespaces

[SeanKilleen]

Did this with the nunit/docs repo.

• Setting up the container (the nunit/docfx-actions container works nicely here)
• Setting up the extensions (we use cSpell, markdown lint, etc.)
• The key -- running docfx serve _site -n "*" which binds correctly to the Codespaces port forwarding.

So cool to see it in action!