Cleanup: reStructuredText markup & minor edits

Hey all - not sure what the history of these pages are, but it seems like the pages' extensions were changed without changing the markup syntax, making some things render incorrectly. Most prominently, links.

I made a few grammar and clarifying edits here and there, as well. I also merged the Git help pages, because (1) Git_Workflow.rst seemed to be an older version of GitHowTo, and (2) Gitkeepingpace and Gitrebasefeaturebranch were really short and covered basically the same information as in GitHowTo.

One long-term goal I have for the summer is to organize the wiki pages a little better to make it easier to access the information. Feedback appreciated!