Add index.html generation for redirecting to ./stable or ./dev

[hyrodium]

This PR solves #937.

[mortenpi]

Thanks @hyrodium, this is great!

I am concerned that this will overwrite user-created files -- there might be some cases where the user wants to maintain their own index.html. So if we want this in a patch release, we should not touch an existing index.html.

However, the URL might change (e.g. dev/ -> v1/ -> v2/). So it is probably good if Documenter updates the file when necessary. I see two options:

1. We could potentially just change the behavior in the next breaking release to always update index.html.
2. Check for an HTML comment in index.html that indicates that the current version is automatically generated, and so can be overwritten.

I am kinda partial two (2), even if it's a little more complicated, since that still allows the user to also manually maintain an index.html file.

[hyrodium]

Thanks for the review!
I've updated the function to check the existence of a comment "<!--This file is automatically generated by Documenter.jl-->" in index.html.

[fredrikekre]

Check for an HTML comment in index.html that indicates that the current version is automatically generated, and so can be overwritten.

This was what I was thinking too, and looks like you implemented that now in the PR so 👍

[hyrodium]

I've fixed the redirected destination, and I think this is ready to merge.

[fredrikekre]

LGTM, can you add an entry to the NEWS.md file?

[hyrodium]

I've updated the CHANGELOG.md. (btw, not NEWS.md:smile:)

[fredrikekre]

.

[fredrikekre]

LGTM, thanks. I'll let @mortenpi look over it and merge.

[mortenpi]

LGTM, thanks @hyrodium and @fredrikekre!