Add rudimentary documentation for AePPL

[rlouf]

In this PR I try to add enough documentation to allow anyone to get started with AePPL.

• [x] Document available distributions
• [x] Document joint_logprob and conditional_logprob
• [x] Document invertible transformations
• [x] Document other transformations
• [x] Add an example with multiple backends
• [x] Demonstrate conditioning on transformed variable
• [x] Add PERT distribution example
• [x] Add mixture example

[codecov[bot]]

Codecov Report

Base: 95.15% // Head: 95.15% // No change to project coverage :thumbsup:

Coverage data is based on head (5201c86) compared to base (b27f95d). Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #213 +/- ## ======================================= Coverage 95.15% 95.15% ======================================= Files 12 12 Lines 2023 2023 Branches 253 253 ======================================= Hits 1925 1925 Misses 56 56 Partials 42 42 ``` | [Impacted Files](https://codecov.io/gh/aesara-devs/aeppl/pull/213?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aesara-devs) | Coverage Δ | | |---|---|---| | [aeppl/censoring.py](https://codecov.io/gh/aesara-devs/aeppl/pull/213/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aesara-devs#diff-YWVwcGwvY2Vuc29yaW5nLnB5) | `96.22% <ø> (ø)` | | | [aeppl/dists.py](https://codecov.io/gh/aesara-devs/aeppl/pull/213/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aesara-devs#diff-YWVwcGwvZGlzdHMucHk=) | `94.50% <ø> (ø)` | | | [aeppl/joint\_logprob.py](https://codecov.io/gh/aesara-devs/aeppl/pull/213/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aesara-devs#diff-YWVwcGwvam9pbnRfbG9ncHJvYi5weQ==) | `95.00% <ø> (ø)` | | 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=aesara-devs). 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=aesara-devs)

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[rlouf]

This is ready for review. I added basic documentation: a list of supported random variables, transforms, scan, mixtures, a home page and a few examples.