Add documentation for custom links

docs/additional-features/custom-links.md

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+# Custom Links
+
+Custom links allow users to place arbitrary hyperlinks within NetBox views. These are helpful for cross-referencing related records in external systems. For example, you might create a custom link on the device view which links to the current device in a network monitoring system.
+
+Custom links are created under the admin UI. Each link is associated with a particular NetBox object type (site, device, prefix, etc.) and will be displayed on relevant views. Each link is assigned text and a URL, both of which support Jinja2 templating. The text and URL are rendered with the context variable `obj` representing the current object.
+
+For example, you might define a link like this:
+
+* Text: `View NMS`
+* URL: `https://nms.example.com/nodes/?name={{ obj.name }}`
+
+When viewing a device named Router4, this link would render as:
+
+```
+<a href="https://nms.example.com/nodes/?name=Router4">View NMS</a>
+```
+
+Custom links appear as buttons at the top right corner of the page. Numeric weighting can be used to influence the ordering of links.
+
+## Conditional Rendering
+
+Only links which render with non-empty text are included on the page. You can employ conditional Jinja2 logic to control the conditions under which a link gets rendered.
+
+For example, if you only want to display a link for active devices, you could set the link text to
+
+```
+{% if device.status == 1 %}View NMS{% endif %}
+```
+
+The link will not appear when viewing a device with any status other than "active."
+
+## Link Groups
+
+You can specify a group name to organize links into related sets. Grouped links will render as a dropdown menu beneath a
+single button bearing the name of the group.