Add link from tutorial 6 to tutorial 7.

[petteja]

Dear contributor, what does this PR do?

Adds a link from tutorial 6 to tutorial 7.

Motivation

Previously the sidebar had to be used to get to the next tutorial in the getting started series. That differs from the previous tutorial entries.

Components affected by this PR

Documentation

Does this PR break backwards compatibility?

No

Fixes

More

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This change is 

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