cleanup installation guide formatting

* Fix some formatting issues
* Make capitalization of titles consistent
* Update image that shows how to select branch
gitlab 7.2 requires gitlab shell 1.9.7
Merge pull request #7577 from bbodenmiller/patch-7

[Documentation] GitLab 7.2 requires GitLab shell 1.9.7
make configuration comment spacing consistent
Merge pull request #7581 from bbodenmiller/patch-9

make configuration comment spacing consistent
Merge pull request #7576 from bbodenmiller/patch-5

[Documentation] cleanup installation guide formatting
unify nginx config files