Fix surprise OutputFormat.Rel overwriting

Fix surprise OutputFormat.Rel overwriting

In page.NewOutputFormat, we take an output.Format f and use it to
create a page.OutputFormat. If the format is canonical, we assign
the final OutputFormat's Rel to "canonical" rather than using
f.Rel. However, this leads to unexpected behavior for custom
output formats, where a user can define a "rel" for a format
via the config file.

For example, the standard for "humans.txt" files requires using
rel="author" in HTML "link" elements. Meanwhile, humans.txt is
usually the only format used for its content. As a result, for
Hugo configurations that define a humans.txt custom output format,
Hugo will render "link" elements to content in this format with
rel="canonical," rather than "author" as required by the standard.

This commit changes page.NewOutputFormat to check whether a given
format is user defined and, if so, skips assigning Rel to
"canonical," even if isCanonical is true.

Fixes #8030