Improve Synthetics notifications page structure and clarity

[git-thuerk-done]

What does this PR do? What is the motivation?

[From Docs AI day]

• Rewrites the Synthetics notifications page to improve clarity and task orientation for first-time readers.

Changes:

• Overview: Removes the flat feature list, and instead adds a Next steps section
• Pre-filled monitor messages: Clarifies that defaults require no setup; wraps the five advanced Handlebars tab examples in a collapsible section so they don't overwhelm new readers (Also changed tab headings to organize by Test type as once I have the cdocs pages for the Template variables it will follow the same structure)
• Notification presets: Rewrites the table with "What it shows" and "When to use it" columns; removes redundant intro copy; drops the vague channel comparison table
• Display custom notifications: Removes duplicate intro copy that conflicted with the presets intro
• Further reading: Replaces entries now covered by the new Next steps section with more useful links (monitor downtimes, alerting guide, monitor notifications)
• Next steps: Adds a dedicated section linking to template variables, conditional alerting, advanced notifications, and the alerting guide

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/alicia.scott/synthetics-notification-presets-table/synthetics/notifications/

[brett0000FF]

Curious why you converted the Markdown table to be HTML? We should prefer Markdown tables unless there is a good reason not to use them.

[git-thuerk-done]

It was word wrapping the "preset" column with Markdown for some reason

[brett0000FF]

The old introductory paragraph for the "Display custom notifications" section was removed, so now this ## section jumps straight into the ### with no context. Consider either:

• Adding a one-line intro under "Display custom notifications message", or
• Promoting "Notification presets" to ## and removing the now-empty parent heading

[git-thuerk-done]

good catch, this was certainly a claude thing that I should have noticed it was minimizing too much information!

[iadjivon]

Is this section saying that it will NOT show the @slack-channel channel? If yes, I'd recommend clarifying this in the example.

[iadjivon]

For this table, let me know if this keeps the meaning:

Preset	What it shows	Best for
show_all (default)	All enriched data, metadata, and handles	Email and ticketing channels where full context is useful
hide_handles	All enriched data, but no handles (for example, @slack-channel)	Channels already notified at the monitor level to prevent duplicate alerts
hide_all	Custom message and event link only	Chat channels where you control the full message format

[git-thuerk-done]

Yes I will make that more clear

[iadjivon]

Hi @git-thuerk-done , thanks for this PR! I've added some comments, let me know if you have any questions.