User management documentation

• Adds a new 'Password requirements' section specifying exact allowed special characters and validation rules for user passwords
• Updates user-management documentation to guide users on how to create new user accounts using sign-in and sign-up API calls with example request payloads
• Clarifies that the initially created admin user is the only account able to create other users and explains the permissions of newly created user accounts
• Expands the AI Assistance overview documentation to explain how the AI assistant chat indicates tool call failures by marking tool outputs as errors and displaying error text in the chat interface
• Fixes internal link formatting in the AI Assistance overview documentation to conform to style guide requirements using absolute doc-root paths

This covers 2 commits.

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This was opened from: https://github.com/openops-cloud/openops/releases/tag/untagged-ee71e3c33a6e2bab9edc

The publication for this is: Documentation