[REM] project: remove doc (#879)

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:
- forecast.rst

Author: kbr-odoo

project.rst

@@ -8,4 +8,3 @@ Project
    :titlesonly:
 
    project/tasks
-   project/planning