Update to docfx 2.70.0

[Arhell]

https://github.com/dotnet/docfx/releases/tag/v2.70.0

[SeanKilleen]

Thanks for this @Arhell! Reviewing the release notes re: the breaking changes and updates to consider and then I'll merge this and add some tasks to the docs backlog.

[SeanKilleen]

Notes here for my future reference:

• We'll want to double-check the theme locally and ensure our overrides that remain still make sense
• For convenience, we'll want to update the container's local serve command to add --open-browser per Add open browser options to docfx-serve command dotnet/docfx#8934
• Check the redirects and template usage of them, per Add redirection document type dotnet/docfx#8892