docs(integrations): document two-way ticket sync and webhook setup

[knakul853]

Blocked on

• Merge after two-way ticket sync ships (projectdiscovery/aurora#3606, pdcp-webhook#183, platform-backend#1675, platform-ui#2009); until then the steps describe a toggle users can't see

Change

The ticketing docs only covered creating tickets, so nobody could learn how to make a closed or reopened ticket update the vulnerability.

• New "Two-way sync" section under Ticketing Integrations: what each tracker state does to the vulnerability, how to turn it on, and how to read the webhook status.
• Step-by-step webhook setup for Jira, GitHub, GitLab and Linear in tabs, with the access each needs and a link to the tracker's own guide.
• Screenshot of the settings panel, with the webhook host replaced by a placeholder.

Evidence

• Steps and labels match the UI in platform-ui#2009; state rules match Aurora's sync engine; Jira and GitHub flows were run live while building the feature.
• All four external links return 200.

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Claude-Session: https://claude.ai/code/session_01Ud2FL9XNTahVE1Ke8W6y1M
Claude-Local-Session: claude --resume c4f3697b-a416-4c11-b645-e5dde2109626