Allows using permalink configuration for sections (branch bundles) and
also for taxonomy pages. Extends the current permalink configuration to
be able to specified per page kind while also staying backward compatible:
all permalink patterns not dedicated to a certain kind, get automatically
added for both normal pages and term pages.
Fixes#8523
Primary motivation is documentation, but it will also hopefully simplify the code.
Also,
* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.
Closes#10896Closes#10620
