Remove index.html

[hyrodium]

If #1657 is merged, index.html will be maintained by Documenter.jl.

We have the folllowing two options, and I choose the first one:

• Delete index.html and Documenter.jl will generate this file again.
  • 👍 The author of commit will be Documenter.jl.
  • 👎 We will lose the redirect page in short time.
• Modify index.html and add a comment <!--This file is automatically generated by Documenter.jl--> to be maintaned by Documenter.jl
  • 👍 We don't lost the redirect page.
  • 👎 The author of commit will be human.

[fredrikekre]

I suggest we just add the documenter comment once the other pr is merged instead of deleting this.

[hyrodium]

Thanks for the review. I've updated the file.

[hyrodium]

I think we can merge this PR before #1657.