New doc topic: issues

Replaces https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7326

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/27650

EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1827

What does this MR do?

• Add documentation on issues doc/user/project/issues/index.md - screenshot
  • Add new doc: issues functionalities - screenshot
  • Add new doc: closing issues - screenshot
  • Add new doc: crosslinking issues - screenshot
  • Rewrite and move doc on creating issues from /giltab-basics/ to user/project/issues/ - screenshot
  • Update CE doc's landing page (section "Issues and MRs") - screenshot

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• If working on CE, submit an MR to EE with the changes as well.