[Docs] Add docs about custom Identity providers for Authentication

[viniciusdc]

Fixes | Closes | Resolves #1030

Changes introduced in this PR:

• Fix a broken link from the management page -- related to how to add users to Keycloak
• Added information about Identity Providers and how to use custom them for custom authentication.

Types of changes

What types of changes does your PR introduce?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests?

• Yes
• No

Documentation

Does your contribution include breaking changes or deprecations?
If so have you updated the documentation?

• Yes, docstrings
• Yes, main documentation
• Yes, deprecation notices

[viniciusdc]

I will be merging this as it's only doc-related. (some of the tests are currently failing due to issues with related -- but not touched -- files)