docs(integrations): add Salesforce integration article

[gtsoi]

Summary

• Adds a new support article at administration/advanced/integrations/integrate-salesforce.mdx documenting the Salesforce integration GA'd 2026-05-05 (two-way sync of Contacts and Companies between Salesforce and Business App / Partner Center).
• Documents the Salesforce MCP under AI Capabilities (search Contacts, Companies, Leads with field details and filter support) and clarifies how it differs from the two-way sync.
• Updates the integrations section index to surface Salesforce in the description, capabilities list, and articles list.

Source: Release note: Salesforce Integration

Test plan

• npm run start from docusaurus/, navigate to /administration/advanced/integrations/integrate-salesforce, confirm the page renders, the FAQ <details> blocks expand, and the cross-link to AI Capabilities works
• Confirm the Integrations index page lists Salesforce in the capabilities and articles lists
• npm run build passes with no broken-link warnings
• Verify the in-product menu path matches what users see (release note says Administration → Connections → Browse; existing docs say Administration → Integrations)
• Add screenshots once available (Connect flow, Connection Configuration page) under img/integrations/salesforce/

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