[SEC-33438] Add Linear to security ticketing integrations public documentation

[nsahifa]

What does this PR do? What is the motivation?

Adds Linear as a third ticketing integration on the security ticketing page, at parity with the existing Jira and ServiceNow integrations:

• Lists Linear alongside Jira and ServiceNow in the intro.
• Adds a Linear tab to Set up bidirectional syncing (prerequisites and setup verification).
• Adds a Linear issue collapsible to Create bidirectional tickets (create or attach by URL, synced fields, and the core-status sync note).
• Adds a Linear tab to Manage bidirectional Case Management tickets.
• Adds Jira Status, Linear Issue Key, and Linear Status to the bidirectional facets list.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

• Preview the page at /security/ticketing_integrations/ (branch preview, or make start-no-pre-build locally).
• The Ticketing integration API section is intentionally unchanged: the Linear public API endpoints are not yet published, so no reference links were added.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/nabil.sahifa/docs-linear-security-ticketing/security/ticketing_integrations