documentation: Add User Models and Repositories reference doc

[evansims]

Changes

This PR adds reference documentation for creating custom user models and repositories for use with the Laravel SDK as a new file, docs/User Models and Repositories.md.

References

Jira: SDK-4150

Testing

N/A

Contributor Checklist

• [x] I have read the Auth0 general contribution guidelines
• [x] I have read the Auth0 code of conduct

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1076e9b) 99.75% compared to head (fbae083) 99.75%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #379 +/- ## ========================================= Coverage 99.75% 99.75% Complexity 295 295 ========================================= Files 21 21 Lines 806 806 ========================================= Hits 804 804 Misses 2 2 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `99.75% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#carryforward-flags-in-the-pull-request-comment) to find out more. Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0)

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