Delete or fix and document the generated maintenance Nginx config

[manics]

There's a badly documented @maintenance section in the generated omero-web nginx config. We should either:

• Fix it (currently the error template is in https://github.com/ome/openmicroscopy/blob/v5.6.0/etc/templates/error/maintainance.html instead of this repo)
• Document exactly how to use it
• Delete it completely

[mtbc]

Might be a good opportunity to fix its spelling!

[manics]

See #168