When building the book, mdBook will always generate an `index.html`
page for first page of the book. This meant that we had the same
content available under two different names:
- `welcome.html`: this is what the TOC would link to, and
- `index.html` or simply `/`: this is what search engines link to
Renaming the page and setting up a redirect should fix this confusion.
We still don’t have a good way of avoiding links to the `index.html`
page, but this should fix the first half of the problem.
I tested this for translations as well by building the output in a
subdirectory and serving the parent directory.
Part of #847.
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