Update string title and array source to fix type definition in docs

[Matteo-it]

What This PR Changes

Developing docs 2.0.0 we found some inconsistency with type definition for title and source frontmatter:

• Title should be a string, in one case we have a number;
• Source it's unused in docs (i guess), but anyway we managed it as a string. We have one case where we have an array.

Contribution Guidelines

• I confirm that I have read the contribution guidelines and comply with them.

[jhansson-ard]

@Matteo-it do we need some manul updates of the tutorials as well to check that all titles in the frontmatter are strings?

[Matteo-it]

@Matteo-it do we need some manul updates of the tutorials as well to check that all titles in the frontmatter are strings?

@jhansson-ard No, I saw that the 2 files I edited are the only two that create type warnings. Anyway it works with number title and array source, but is better remove the easy warning when we found them :)

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[jhansson-ard]

LGTM