Feat/restrict serverurl path

[jmikrut]

Description

1. Ensures that serverURL does not contain a path, and validates accordingly using existing Joi schema validation.
2. Updates docs to be more explicit regarding serverURL.
3. Moves paths from exposed Payload config type to internal Payload config type.

• I have read and understand the CONTRIBUTING.md document in this repository

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• Existing test suite passes locally with my changes