Update Timeshift docs with "before" functions deprecation

[brebernjak]

What does this PR do? What is the motivation?

Reflecting some product changes in the documentation:

• calendar_timeshift is now available everywhere
• explain better the arguments and their possible values
• reorder the page content to make calendar_timeshift more visible
• "before" functions are being deprecated (except hour_before)

Merge instructions

• [x] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/bartol/timeshift-docs/dashboards/functions/timeshift