docs: user-guide/helpers.md → lib/index.md

Move the user-guide to the new lib reference docs.

Added a MDBook redirect entry.

Updated man docs.

docs/mdbook/SUMMARY.md

@@ -5,7 +5,6 @@
 # User guide
 
 - [Installation](./user-guide/install.md)
-- [Helpers](./user-guide/helpers.md)
 - [FAQ](./user-guide/faq.md)
 - [Configuration examples](./user-guide/config-examples.md)
 - [Lazy Loading](./user-guide/lazy-loading.md)

docs/mdbook/book.toml

@@ -10,6 +10,11 @@ site-url = "@SITE_URL@"
 additional-js = ["theme/pagetoc.js"]
 additional-css = ["theme/pagetoc.css"]
 
+# Redirect targets must be relative to their origin;
+# absolute paths can break with different baseHrefs.
+[output.html.redirect]
+"/user-guide/helpers.html" = "../lib/index.html" # Since #3049
+
 [output.html.fold]
 enable = true
 level = 0