[MOV] calendar: "Calendar" app from "Misc" to "Productivity"

To improve the structure of the documentation, the "Miscellaneous"
section should be limited as much as possible.

This commit:
- renames "Calendars" into "Calendar" (as the app name)
- moves the app from the "Miscellaneous" section to the "Productivity"
  section
- fixes the wrong file structure to follow the toc
  (e.g. general/calendars/outlook/outlook_calendar -> productivity/calendar/outlook)

task-3217827

closes odoo/documentation#4109

Signed-off-by: Castillo Jonathan (jcs) <jcs@odoo.com>

content/applications/productivity/calendar.rst

@@ -0,0 +1,11 @@
+:nosearch:
+
+========
+Calendar
+========
+
+.. toctree::
+   :titlesonly:
+
+   calendar/outlook
+   calendar/google