Getting started guide should mention dependency cycles

This page has very useful information on dependency cycles: https://grafikrobot.github.io/boost_lib_stats/

Many people seem to think you can't download and install individual libraries because boost has dependency cycles but this is no longer the case since boost 1.77.

This information should be included as an admonition in the "individual modules" section of the "getting started" page.

That page also has a somewhat useful [popularity metric](https://grafikrobot.github.io/boost_lib_stats/#_popularity) we can use to guide us when writing about use cases. We have to take that with a grain of salt though because it's only based on GitHub "watchers", "forks", and "stars". A better measure would probably be just shallow cloning the top X (say 10000) C++ projects with the most stars (some other metric (?)) and checking what boost libraries they are using:

```bash
API_URL="https://api.github.com/search/repositories?q=language:C++&sort=stars&order=desc&per_page=10000"
REPO_URLS=$(curl -s "$API_URL" | jq -r '.items[].clone_url')
for URL in $REPO_URLS; do
git clone --depth 1 "$URL"
done
```