To link to or from a [[SubPage]], you can normally use a regular
[[WikiLink]] that does not contain the name of the parent directory of
the [[SubPage]]. Ikiwiki descends the directory hierarchy looking for a
page that matches your link.
For example, if FooBar/SubPage links to "OtherPage", ikiwiki will first
prefer pointing the link to FooBar/SubPage/OtherPage if it exists, next
to FooBar/OtherPage and finally to OtherPage in the root of the wiki.
Note that this means that if a link on FooBar/SomePage to "OtherPage"
currently links to OtherPage, in the root of the wiki, and FooBar/OtherPage
is created, the link will change to point to FooBar/OtherPage. On the
other hand, a link from BazBar to "OtherPage" would be unchanged by this
creation of a [[SubPage]] of FooBar.
You can also specify a link that contains a directory name, like
"FooBar/OtherPage" to more exactly specify what page to link to. This is
the only way to link to an unrelated [[SubPage]].
You can use this to, for example, to link from BazBar to "FooBar/SubPage",
or from BazBar/SubPage to "FooBar/SubPage".
You can also use "/" at the start of a link, to specify exactly which page
to link to, when there are multiple pages with similar names and the link
goes to the wrong page by default. For example, linking from
"FooBar/SubPage" to "/OtherPage" will link to the "OtherPage" in the root
of the wiki, even if there is a "FooBar/OtherPage".
