docs: Redirect reference/ -> api/

Author: tony

Diff for: docs/redirects.txt

@@ -6,3 +6,12 @@
 "sessions.md" "reference/sessions.md"
 "api.md" "reference/index.md"
 "pytest-plugin.md" "pytest-plugin/index.md"
+"reference/common.md" "api/common.md"
+"reference/constants.md" "api/constants.md"
+"reference/exceptions.md" "api/exceptions.md"
+"reference/index.md" "api/index.md"
+"reference/panes.md" "api/panes.md"
+"reference/properties.md" "api/properties.md"
+"reference/servers.md" "api/servers.md"
+"reference/sessions.md" "api/sessions.md"
+"reference/windows.md" "api/windows.md"