Update slide deck example in Markdown guide

Why is this change being made?

This MR changes the slide deck linked from the Markdown Guide, as the current one doesn't appear for non-GitLab users (check this by opening the page in incognito mode).

The link used in this iteration opens this deck and was provided by @luke.

Review app: https://patch-19826111962.about.gitlab-review.app/handbook/engineering/ux/technical-writing/markdown-guide/index.html#google-slides

Close #6806 (closed)

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what