Docs: Add Veeva Vault Integration Developer Documentation

[fderuiter]

Drafted developer documentation for the upcoming Veeva Vault integration feature. Created docs/veeva_integration.rst and updated docs/index.rst to include it in the table of contents. The documentation covers prerequisites, configuration, usage, and data mapping.

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PR created automatically by Jules for task 10062056888465793085 started by @fderuiter

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