[FEAT] Add description in the 'blog' and 'news' pages

[tbouffard]

covers #187

[github-actions]

♻️ PR Preview e4453ee has been successfully destroyed since this PR has been closed.

_{🤖 By surge-preview}

[csouchet]

There is a problem. The cards take the same witdh that this text.

[csouchet]

Also, I would prefer to simplify this code and the rendered objects in the DOM, like:

Suggested change

	<Heading
	as="p"
	color="primary"
	fontSize={[2, 4]}
	mb={[3, 5]}
	textAlign="center"
	style={centerHorizontally}
	>
	<Text width={[400, 600, 700]} key="presentation">
	{description}
	</Text>
	</Heading>
	<Text
	as="p"
	width={['85%', '80%', '70%']}
	color="primary"
	fontSize={[2, 4]}
	mb={[3, 5]}
	textAlign="center"
	style={centerHorizontally}
	>
	{description}
	</Text>

It also is recommended to use 'rem' or '%' in place of 'px' in CSS.
It prevents some effect like:

px	%

[tbouffard]

I am ok for using percentage instead of other configuration. I took the width values from existing we had from the site, so this should be applied everywhere.

[tbouffard]

done in e4453ee

[csouchet]

In a future PR, I'll change all the unit for the different CSS styles 😉

[csouchet]

See my previous comment https://github.com/process-analytics/process-analytics.dev/pull/226/files#r757508818