Simple, clean document URLs, e.g. docs.decidim.org/whitepaper

[gmarpons]

For this to work, we need:

1. An index.adoc file in every document that needs a clean URL or, alteranitevely, set a start page (see https://docs.antora.org/antora/2.0/component-descriptor/#start-page-key) for those documents.
2. Enable a redirection mechanism that adds version and language segments (via Antora or, alternatively, Netlify). This should work automatically, we don't want to manually maintain a redirections file.
3. Indexify either with Antora or Netlify (but check interactions with point 1).