Standardize on wikilink (#39)

Not `wiki-link`, nor `wiki link`

https://en.wikipedia.org/wiki/Help:Link#Wikilinks

docs/features/wikilinks.md

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+# Wikilinks
+
+Wikilinks are the internal links that connect the files in your knowledge base.
+
+To create a wikilink use `[[` and type the name of another file in your repo, for example [[graph-visualization]]
+
+You can also create a [[placeholder]].
+A placeholder is a wikilink that doesn't have a target file and a link to a placeholder is styled differently so you can easily tell them apart.
+They can still be helpful to highlight connections.
+
+Open the graph with `Foam: Show Graph` command, and look at the placeholder node.
+
+Remember, with `CTRL/CMD+click` on a wikilink you can navigate to the note, or create it (if the link is a placeholder).