Docs: refactor content of Issue Boards

The problem

• Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21905

We need to improve the Issue Boards documentation:

• Better descriptions
• Update all screenshots / gifs to match the current UI
• Maybe move the doc into /issues/
• Maybe split the doc into 2: one for feature descriptions, another one with the how-tos
• Address https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21905#note_104245842
• Address https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21905#note_104246048
• Read the whole content looking for improvements on descriptions and how-tos

New doc or update?

• New documentation
• Update existing documentation

Priority

Issue Boards are very popular and people love them, so I'll give this a gitlab-ce~5796837 as initial prioritization, but it's subjected to change according to our priorities.

Pinging myself to keep this on my radar: @marcia