Update API documentation Leads & Billing Activities

[tri-boompay]

ticket: https://boompay.atlassian.net/browse/BSCPF-3624
Add API documentation for Leads and Billing Activities endpoints.

Added endpoints:

Leads:

• GET/POST /partner/v1/leads
• GET/PATCH /partner/v1/leads/{id}
• POST /partner/v1/leads/{id}/issue_access_token
• POST /partner/v1/leads/{id}/apply
• GET /partner/v1/leads/activity_logs
• GET /partner/v1/leads/{id}/activity_logs
• POST /partner/v1/leads/{id}/verifications/identity/expire

Billing Activities:

• GET /partner/v1/billing/statements
• GET /partner/v1/billing/statements/{id}
• GET /partner/v1/billing/statements/{id}/activities
• GET /partner/v1/billing/activities

Also documented additional Partner API endpoints: roles, property_groups, customers (with Plaid integration), ledgers, and enrollments.