Merge information about Gerrit, reviewing / testing patches and improving patches

[sypets]

Currently, there is some overlap of information, specifically in:

• the chapter on Gerrit: https://docs.typo3.org/typo3cms/ContributionWorkflowGuide/Gerrit/Index.html
• the chapter "Livecycle of a patch" in the Appendix: https://docs.typo3.org/typo3cms/ContributionWorkflowGuide/Appendix/LifeOfAPatch.html
• in the wiki page: https://wiki.typo3.org/Gerrit_Review_Workflow (see also Contrib Guide: Migrate things from TYPO3 wiki #58)

This information could be merged into one chapter and duplicate information removed.

Additionally, the page on Gerrit might be changed to be more consistent with the rest of the guide, specifically:

• The rest of the guide is structured in a way that the information is supplied in the order that a contributor needs it. In the case of the Gerrit chapter, it contains a lot of info on reviewing a patch, which most likely will be done after someone submits a patch. Currently the reader is confronted with a lot of details before he needs it. Most likely, they just want to create a patch. So it is probably better to move this chapter to after "Fixing a bug"