hugo: bump version to 0.147.8, address deprecations

This change bumps the Hugo version used to build web pages to v0.147.8
and addresses build issues introduced by functions deprecated in newer
hugo versions:
- .Site.IsMultiLingual => hugo.IsMultilingual
- resources.ToCSS => css.Sass

Templates and partials that use deprecated functions are now overridden
in layout/.

Lastly, symlinks from content/ to the Readme and to top level specs/
were removed as Hugo does not support symlinks anymore, and replaced by
"mounts" in config.toml. Consequently, all symlinks used in specs/ were
also removed (without replacement).

The Hugo version was bumped in gh-pages.yml.

Signed-off-by: Thilo Fromm <[email protected]>

Author: t-lo

.github/workflows/gh-pages.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Setup Hugo
         uses: peaceiris/actions-hugo@v2
         with:
-          hugo-version: '0.104.3'
+          hugo-version: '0.147.8'
           extended: true
 
       - name: Build