[15.0+] Missing group node inside notebook page nodes

[delvsola]

Some form views examples are missing the <group>...</group> tags inside <notebook><page>...</page></notebook> nodes.

This leads to an improper rendering of the fields inside the view and is misleading for newer developers figuring out form views.

Occurrences of this issue:

documentation/content/developer/tutorials/getting_started/07_basicviews.rst

Lines 101 to 105 in f94085f

	<notebook>
	<page string="Description">
	<field name="description"/>
	</page>
	</notebook>

documentation/content/developer/tutorials/backend.rst

Lines 375 to 379 in 05637af

	<notebook colspan="4">
	<page string="Description">
	<field name="description" nolabel="1"/>
	</page>
	</notebook>

[AntoineVDV]

Hi @delvsola, would you mind creating the PR? If needed, you'll find the steps to build the doc locally in the README file ;)