Move Issues and Merge requests to level 2

Now that we have:

• Four- and five-level navigation: !1625 (merged).
• More level-one items on the navigation: !1482 (merged).

We need to trim some of our level 1 items to stop the initial menu from overwhelming.

Issues and Merge requests are a good candidate because:

• They are scoped to projects, so naturally have a home under "Projects".
• The files themselves are stored under .../project/.
• They used to be that way: https://docs.gitlab.com/12.10/ee/user/project/.

Issues:

• Before: https://docs.gitlab.com/ee/user/project/issues/
• After: http://eread-demote-issues-merge-requests.178.62.207.141.nip.io/ee/user/project/issues/

Merge requests:

• Before: https://docs.gitlab.com/ee/user/project/merge_requests/
• After: http://eread-demote-issues-merge-requests.178.62.207.141.nip.io/ee/user/project/merge_requests/

Note that the Burndown and burn charts topic can live at level 4:

• Before (at level 3): https://docs.gitlab.com/ee/user/project/milestones/burndown_and_burnup_charts.html
• After: http://eread-demote-issues-merge-requests.178.62.207.141.nip.io/ee/user/project/milestones/burndown_and_burnup_charts.html