Consistent URL endings

We are mixing `.html` with more modern endling-less URLs, which leads to confusion in search engines as they are indexed as separate pages.

We should decide which version we want to stick to, make it consistent and ideally enforce it.

Example:
* https://precice.org/about and https://precice.org/about.html serve the same `about.html`.
* https://precice.org/ , https://precice.org/index and https://precice.org/index.html serve the `index.html`

What we need to choose:
1. without ending https://precice.org/about
2. with ending https://precice.org/about.html

What we need to do after the decision:
* Adjust canonical metadata
* Fix sitemap (may require changing permalinks)
* Fix links