doc: Link to Documentation Generation from Documentation Guidelines

Gerson Fernando Budke missed the Documentation Generation page. Nice to
link to it from the Documentation Guidelines page, in case people are
looking for documentation information and find it first.

Maybe the two pages could be put under a common Documentation section
too, though it's nice to have Documentation Generation clearly visible
in the top-level User Guides index.

Co-authored-by: Gerson Fernando Budke <nandojve@gmail.com>
Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>