Add portuguese translation of index.html

**Work in progress. Don't merge it yet.**

This implement the first suggestion of #602 because `index.html` uses `_includes/setup.html` and I didn't find one way to make `translations/pt/index.html` uses `translations/pt/_includes/setup.html` or something similar.

## Long Description (and TODO list)

- [x] Replace `index.html` with `index-en.html`
- [x] Create `index.html` with redirect to `index-en.html`
- [x] Create `index-pt.html`
- [ ] Update documentation
- [ ] Replace `_includes/setup.html` with `_includes/setup-en.html`
- [ ] Create `_includes/setup-pt.html`