Add threshold system usage examples with wildcards

[lunkwill42]

Is your feature request related to a problem? Please describe.
The NAV documentation does not contain a sufficiently detailed usage guide for configuring threshold rules.

Although the threshold rule editor seems like it is intuitive, with links from several parts of the NAV UI to add new threshold rules, it is not obvious that wildcards are supported in metric paths (Graphite specific). Users who aren't aware of this, tend to create individual rules for each metric they wish to observe, or they forego the rules entirely because it seems tedious to create the same rule for many sensors, for example.

Describe the solution you'd like
An updated guide in NAV's official documentation, describing how to use the threshold rule editor, and with examples of how to create rules that include wildcards in metric paths.