Hugo v0.121.1: fix warnings

[deining]

When building the example site with latest hugo version v0.121.1, several warnings are issued.

WARN  Page '/overview/configuration/' not found in 'posts/migrate-from-jekyll.md'
WARN  Page '/layout/templates/' not found in 'posts/migrate-from-jekyll.md'
WARN  Page '/doc/shortcodes/' not found in 'posts/migrate-from-jekyll.md'
WARN  Page '/layout/variables' not found in 'posts/goisforlovers.md'
WARN  Page '/layout/functions' not found in 'posts/goisforlovers.md'
WARN  Page '/content/front-matter' not found in 'posts/goisforlovers.md'
WARN  Expand shortcode is deprecated. Use 'details' instead.

This PR fixes all these warnings except for the warning concerning the expand shortcode.

[alex-shpak]

Hi!
Thanks for the PR, tho, I left these warning intentionally to show work of BookPortableLinks param :)