docs: Prepare for new sub tree

See #11925

docs/content/en/commands/hugo_gen_doc.md

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----
-title: "hugo gen doc"
-slug: hugo_gen_doc
-url: /commands/hugo_gen_doc/
----
-## hugo gen doc
-
-Generate Markdown documentation for the Hugo CLI.
-
-### Synopsis
-
-Generate Markdown documentation for the Hugo CLI.
-			This command is, mostly, used to create up-to-date documentation
-	of Hugo's command-line interface for https://gohugo.io/.
-
-	It creates one Markdown file per command with front matter suitable
-	for rendering in Hugo.
-
-```
-hugo gen doc [flags] [args]
-```
-
-### Options
-
-```
-      --dir string   the directory to write the doc. (default "/tmp/hugodoc/")
-  -h, --help         help for doc
-```
-
-### Options inherited from parent commands
-
-```
-      --clock string               set the clock used by Hugo, e.g. --clock 2021-11-06T22:30:00.00+09:00
-      --config string              config file (default is hugo.yaml|json|toml)
-      --configDir string           config dir (default "config")
-      --debug                      debug output
-  -d, --destination string         filesystem path to write files to
-  -e, --environment string         build environment
-      --ignoreVendorPaths string   ignores any _vendor for module paths matching the given Glob pattern
-      --logLevel string            log level (debug|info|warn|error)
-      --quiet                      build in quiet mode
-  -s, --source string              filesystem path to read files relative from
-      --themesDir string           filesystem path to themes directory
-  -v, --verbose                    verbose output
-```
-
-### SEE ALSO
-
-* [hugo gen](/commands/hugo_gen/)	 - A collection of several useful generators.
-