Set absolute paths in 404 page

Signed-off-by: George Melikov <mail@gmelikov.ru>

docs/conf.py

@@ -46,6 +46,7 @@ extensions = [
     'sphinx.ext.intersphinx',
     "sphinx_issues",
     "sphinx_rtd_theme",
+    "notfound.extension"
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -214,3 +215,7 @@ issues_github_path = "openzfs/zfs"
 issues_uri = "https://github.com/openzfs/zfs/issues/{issue}"
 issues_pr_uri = "https://github.com/openzfs/zfs/pull/{pr}"
 issues_commit_uri = "https://github.com/openzfs/zfs/commit/{commit}"
+
+# Get absolute paths in 404
+notfound_no_urls_prefix = True
+notfound_url_prefix = '/openzfs-docs'