CTRT Restructure contribution analytics

What does this MR do?

• Reorders the "Change time period" section after "Sort contributions". The thinking behind this change is that the selected time frame applies to all charts and the table; the View and Sorting sections refer to the contribution analytics data, in the order they're displayed on the page (charts followed by a table), so it makes more sense to list them in this order.
• Makes the sections that refer to interacting with the charts subsections of "View contribution analytics", and adds a subsection with instructions for zooming in on charts.
• Changes the verb form of headings.

	Before	After
Page	https://docs.gitlab.com/ee/user/group/contribution_analytics/index.html	https://main-ee-111294.docs.gitlab-review.app/ee/user/group/contribution_analytics/
Reading score	7.08	6.69

Other proposed changes (in this or a follow-up MR):

• Remove screenshots.
• Edit the language (e.g. rephrase sentences to follow the reason-location-action structure).
• Update instructions/information in the sections "Change time period" and "Sort contributions".

Related issues

technical-writing#749 (closed)

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