Issue #548: Refactor the getting started vignette

[seabbs]

Description

This PR closes #548 #68 and #403 by refactoring the getting started vignette to:

• Reduce the focus on simulating data
• Make the entry to the package friendlier
• Show a comparison of fitting a naive and a marginal model
• Show how to get out the estimated parameters and plot them vs the truth

In developing this PR I have explored the predict workflow and corrected the following issues:

• bounds were not set for mu parameters in custom families
• I fixed a bug in predict_delay_parameters where it couldn't detect brms families when used directly.

I will open an issue for some thoughts on the current postprocess workflow as it feels like there are too many paths to take and some common tasks require to much work.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.66%. Comparing base (f08a6ce) to head (f110d7b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #549      +/-   ##
==========================================
+ Coverage   95.59%   95.66%   +0.07%     
==========================================
  Files          17       17              
  Lines         998     1015      +17     
==========================================
+ Hits          954      971      +17     
  Misses         44       44              

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[seabbs]

LGTM from a read of the code (self-review). Waiting on the rendered site for final checks.

[seabbs]

The html looks good so I think this is good to go.