Create Developers top level menu

This introduces a new top-level "Developers" menu. It takes the following from the Athena menu:

• all of the material in the "Athena developers" sub-menu of Athena
• the Git workflow (including the Git tutorial underneath it)
• IDE integration

Other menus are left alone, with the exception that the "Software infrastructure" menu is renamed "Shifts and infrastructure".

Other changes:

• the ordering of the items in the developers' menu is slightly changed
• the persistency section of the developers' tutorial is moved to its correct place
• an evidently orphaned part of the Git tutorial is reunited with the rest of the material (see #47 (closed))

Closes #47 (closed).