Reorganise user guide section

[penelopeysm]

WIP

To avoid breakage when deployed, need to check that all of these links work:

• https://turinglang.org/docs/pr-previews/578/tutorials/00-introduction
• https://turinglang.org/docs/pr-previews/578/tutorials/01-gaussian-mixture-model
• https://turinglang.org/docs/pr-previews/578/tutorials/02-logistic-regression
• https://turinglang.org/docs/pr-previews/578/tutorials/03-bayesian-neural-network
• https://turinglang.org/docs/pr-previews/578/tutorials/04-hidden-markov-model
• https://turinglang.org/docs/pr-previews/578/tutorials/05-linear-regression
• https://turinglang.org/docs/pr-previews/578/tutorials/06-infinite-mixture-model
• https://turinglang.org/docs/pr-previews/578/tutorials/07-poisson-regression
• https://turinglang.org/docs/pr-previews/578/tutorials/08-multinomial-logistic-regression
• https://turinglang.org/docs/pr-previews/578/tutorials/09-variational-inference
• https://turinglang.org/docs/pr-previews/578/tutorials/10-bayesian-differential-equations
• https://turinglang.org/docs/pr-previews/578/tutorials/11-probabilistic-pca
• https://turinglang.org/docs/pr-previews/578/tutorials/12-gplvm
• https://turinglang.org/docs/pr-previews/578/tutorials/13-seasonal-time-series
• https://turinglang.org/docs/pr-previews/578/tutorials/15-gaussian-processes
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-00-getting-started
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-08-using-turing
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-09-using-turing-advanced
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-10-using-turing-autodiff
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-11-using-turing-dynamichmc
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-12-using-turing-guide
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-13-using-turing-performance-tips
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-15-using-turing-sampler-viz
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-16-using-turing-external-samplers
• https://turinglang.org/docs/pr-previews/578/tutorials/docs-17-mode-estimation
• https://turinglang.org/docs/pr-previews/578/tutorials/usage-custom-distribution
• https://turinglang.org/docs/pr-previews/578/tutorials/usage-generated-quantities
• https://turinglang.org/docs/pr-previews/578/tutorials/usage-modifying-logprob
• https://turinglang.org/docs/pr-previews/578/tutorials/usage-probability-interface

[github-actions]

Preview the changes: https://turinglang.org/docs/pr-previews/578
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[penelopeysm]

Merging as this is just a follow up to #572, which was already discussed

[yebai]

The name "User Tips" is a bit informal. Would "User Guide" or "User Reference" be a better section name?

[penelopeysm]

Didn't expect to see you here @yebai 👀 Yes, definitely happy to go with 'User Guide'