Consider building doc with docfx

[CarlVerret]

I consider this file as build-artifact, so it doesn't need to be checked in.

Not the scope for this PR, but as idea: With https://dotnet.github.io/docfx you can create a static documentation site, which could be hosted here in the GH-pages to this project. Additional markdown files, etc. can also be incorporated into that doc-build. It's quite easy to automate and push the changes to GH-pages by CI.

Originally posted by @gfoidl in https://github.com/CarlVerret/csFastFloat/pull/76#discussion_r593024583