Adapt to new template system in Hugo v0.146.0

[chalin]

For details, see:

• https://gohugo.io/templates/new-templatesystem-overview/
• https://discourse.gohugo.io/t/documentation-of-the-new-template-system-in-hugo-v0-146-0/54686

Tasks

• Move layouts/_default/_markup one level up, and adjust UG #2246
• Add underscore to partials and shortcodes directory names #2247
• Drop _internal prefix from template paths #2250
• User guide: fix wrong file name, update version information #2251
• Moving some files out of layouts/_default, and using all.html #2255
• Move taxonomy and terms out of _default #2257
  Changelog updates for layout refactorings #2260
• Update min Hugo version in theme config, and some UG copyedits #2267

Related refactorings

• Move new content-after-header hook into partials folder, add UG entry #2256
  • Seems like I'll have to revert this in light of https://discourse.gohugo.io/t/can-partials-be-page-path-specific-too/54836
• Rename layout fragment to include _td- filename prefix #2259
• Changelog updates for layout refactorings #2260
• Make content-after-header a layout fragment again #2261
• Consider using .Render for the render-heading hook inclusion?

[deining]

WIth the change to the new template system, this line is not valid any more:

docsy/theme.toml

Line 9 in 8e52291

min_version = "0.110.0"

We should also scan the user guide and the example site for any statements regarding the require minimum version of Hugo.

• The section Deployment with Netlify states an outdated hugo version.
• ...

I think we have to raise minimum required hugo versionto v0.146.0 now.