Improve documentation of analyzers and fixers

[mikkelbu]

The initial version of the documentation has been implemented in #139. There are still some initial work to be made:

• Add descriptions of all analyzers, see discussion in Tests checking documentation #139
• Add more telling titles to the analyzers, so we get a better TOC, see e.g. https://github.com/nunit/nunit.analyzers/blob/master/documentation/index.md
• Add help links in the analyzers, so that we can navigate from VS to the documentation (and also so that the link from index.md to the analyzer works). Will be done in Add helplinks pointing to analyzer docs #140.