XIOS: documentation

We recently added a https://github.com/nextsimhub/nextsimdg/blob/develop/docs/xios.rst page to the docs. However, it's missing quite a bit of content and would benefit from the following:

* Motivation and background; general idea of what we want to achieve.
* How to build with XIOS support.
* XIOS concepts (calendar, domain, file, etc.).
* Extend developer notes.