Moving some files out of layouts/_default, and using all.html

[chalin]

• Contributes to Adapt to new template system in Hugo v0.146.0 #2243
• Moves most files out of layouts/_default
• Replaces layouts/{home,single.list}.html` by layouts/all.html`
• Updates the UG accordingly.
• Adds temp link-checker ignore rules for moved pages
• Drops inlined content of baseof.html, since it has changed, and interested readers can just click on the link.

Only non-whitespace changes are to the pages that have prose updates:

$ (cd userguide/public && g diff -bw --ignore-blank-lines) | grep ^diff | grep -Ev "_print|\.xml|refcache"
diff --git a/docs/adding-content/lookandfeel/index.html b/docs/adding-content/lookandfeel/index.html
diff --git a/docs/get-started/other-options/index.html b/docs/get-started/other-options/index.html
diff --git a/project/changelog/index.html b/project/changelog/index.html