Fix warning: '.File.UniqueID on zero object' (#1007)

[deining]

This PR closes #1007 (see my analysis there). The fix is quite easy, all we have to do here is to put a {{ with }} statement around the offending code line 8:

7  {{ with .Parent.File -}}
8     {{ $pages = (where $pages "Parent.File.UniqueID" "==" $parent.File.UniqueID) -}}
9  {{ end -}}

Also I did some code cleanup in order to prevent superfluous whitespace and lines.
I did a diff of the published user guide, and there are no changes caused by this PR other than white space cleanup.

[chalin]

Wonderful, thank you! See inline comment for one nit.

[chalin]

The test failure is due to a flaky external link check (which should be fixed via #1790).