Add more links in the Features/Integration testing documentation
Add more links in the Features/Integration testing documentation

- Move the rspec-rails feature spec link to the Features/Integration
  section
- Mention and link to capybara and poltergeist there as well

[ci skip]

stoplightio/gitlabhq,t-zuehlsdorff/gitlabhq,LUMC/gitlabhq,dreampet/gitlab,LUMC/gitlabhq,axilleas/gitlabhq,t-zuehlsdorff/gitlabhq,t-zuehlsdorff/gitlabhq,iiet/iiet-git,htve/GitlabForChinese,mmkassem/gitlabhq,darkrasid/gitlabhq,dreampet/gitlab,stoplightio/gitlabhq,jirutka/gitlabhq,darkrasid/gitlabhq,mmkassem/gitlabhq,darkrasid/gitlabhq,dplarson/gitlabhq,dplarson/gitlabhq,LUMC/gitlabhq,htve/GitlabForChinese,stoplightio/gitlabhq,iiet/iiet-git,axilleas/gitlabhq,darkrasid/gitlabhq,dplarson/gitlabhq,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,stoplightio/gitlabhq,htve/GitlabForChinese,axilleas/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,iiet/iiet-git,jirutka/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,htve/GitlabForChinese,mmkassem/gitlabhq,LUMC/gitlabhq,jirutka/gitlabhq,dreampet/gitlab