Add documentation

[dalonsoa]

Problem

At the moment, all documentation is contained in the README. That worked so far because pycsvy was not doing much. But now, available in both pypi and conda, supporting multiple formats, with multiple input arguments and the availability of header validators - not event mentioned in the README - we need proper documentation in place.

Solution

Implement the documentation using MkDocs and populate it with the relevant sections.

• Implement the standard MkDocs infrastructure #171
• Add installation instructions #172
• Add section on using the readers/writers #173
• Add section on using the validators #174
• Add the API documentation #175