[DOCS-12977] cleanup of Synthetics Notifications

[git-thuerk-done]

What does this PR do? What is the motivation?

This PR doesn't update much content, it includes:

• Adding H3's to the conditional alerting page so those sections can be linked to
• Linking to sections on the conditional alerting page from the template variables page where relevant
• Linking to the API error codes page where relevant
• Adds a new example on the conditional alerting page
• Changes the code samples from mustache to shell to it can be copied
• Converting a lengthy sentence to a table to it is easier to parse

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/aliciathuerk/update-api-failure/synthetics/notifications/conditional_alerting
• https://docs-staging.datadoghq.com/aliciathuerk/update-api-failure/synthetics/notifications/template_variables

[maycmlee]

LGTM, just a small trailing whitespace nit!