Doc suggestions

[Christovis]

This PR contains the following changed to the RTD:

• docs/index.rst: reformulation and spelling
• docs/intro.rst: spelling

Furthermore, I was thinking about the following

**Getting Started**
- Installation
- Motivation  (place this above Installation)
- License

**Documentation**
- Data sources
- Git as data source  (make this part of 'Data sources')
- API Documentation

(Should we give another bullet point to the preprocessing scripts for analysis and visualisation?)

**Community**
- Contribution (add this, to info on communication channels, code formatting, etc.)
- Data Access
- Release notes

[sbenthall]

Merging this and moving your unimplemented ideas to an issue

[sbenthall]

499