Need to write index.html in deploydocs

[asinghvi17]

We don't do that currently, Documenter does it here

https://github.com/JuliaDocs/Documenter.jl/blob/3ada4620e0952c9e2a0697e2bdb0b9ab0e28af47/src/deploydocs.jl#L547-L549

the result is that new sites using Vitepress will not receive the correct result.