[REM] planning: remove docs (#880)

This page documents a straightforward flow users can easily figure out without dedicated documentation. Besides, screens are frequently reworked, requiring to maintain the related page closely. This commit then removes a low-value page to reduce the technical debt and allow focusing on actually useful documentation pages.
Doc removed:
- duplicate_a_planning.rst
- send_planned_shifts.rst

Author: kbr-odoo

_static/banners/planning.png

@@ -60,7 +60,6 @@ Applications
    subscriptions
    project
    timesheets
-   planning
    fsm
    sign
    point_of_sale