RFC: add the log-logistic distribution

## Checklist

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- [x] Read and understood the [Code of Conduct][code-of-conduct].
- [x] Searched for existing issues and pull requests.
- [x] If this is a general question, searched the [FAQ][faq] for an existing answer.
- [x] If this is a feature request, the issue name begins with `RFC:`.

## Description

> Description of the issue (or feature request).

This RFC proposes adding the [log-logistic distribution](https://en.wikipedia.org/wiki/Log-logistic_distribution) (aka Fisk distribution). Similar to other distribution namespaces, adding support for the log-logistic distribution would entail a family of packages for evaluating the PDF, CDF, quantile function, mean, median, etc.

Namespace: `@stdlib/stats/base/dists/log-logistic`
Reference: [`@stdlib/stats/base/dists/*`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/stats/base/dists)

## Related Issues

> Does this issue (or feature request) have any related issues?

No.

## Questions

> Any questions for reviewers?

No.

## Other

> Any other information relevant to this issue (or feature request)? This may include screenshots, references, stack traces, sample output, and/or implementation notes.

Reference implementations:

- [SciPy](https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.fisk.html#scipy.stats.fisk)

[code-of-conduct]: https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md

[faq]: https://github.com/stdlib-js/stdlib/blob/develop/FAQ.md

Start by reviewing the existing distribution packages under lib/node_modules/@stdlib/stats/base/dists and the proposed namespace @stdlib/stats/base/dists/log-logistic. Use the SciPy fisk implementation and the linked distribution modules as references. Done means adding the log-logistic distribution family, including PDF, CDF, quantile function, mean, and median support.

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