fdo#72420: Startcenter: make all helpid point to startcenter's main help page

Until the specific ui elements don't have own help pages.
With this we can avoid to open an empty help page by pushing F1 or
clicking on Help button.
Four ui elements' helpid is not set yet: 'Extensions' button, 'Help' button,
recent docs view.

(cherry picked from commit 0f7b9d00562b8d87d1fb12bb21555907d3f0e874)

Conflicts:
	source/text/shared/guide/startcenter.xhp

Change-Id: Ic0c1bd1dba4c08fd6b3d9daf8fbc9e622a54b6f2
Reviewed-on: https://gerrit.libreoffice.org/7396
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>