Update histogram best practices and metric types documentation for native histograms

[beorn7]

Fixes #2803.

With this update, the best practices page about histograms and summaries and the concepts page about metric types finally takes native histograms into account.

While working on this, it occured to me that the best practices page about histograms and summaries would probably benefit from a more fundamental rewrite, based on the user experience of the last decade. Some of the focal points of the document seem outdated from today's perspective, while other topics might be missing. (I find the focus on Apdex score a bit weird by now, and also the detailed error analysis might not appeal to a broad audience…) However, a complete rewrite would have taken a lot of time, and I did not want to let our users wait for even longer. So I went for this incremental update – which shouldn't prevent anybody from a thorough rewrite in the future.

Note that I used this opportunity to replace the term "client library" with "instrumentation library". I always thought that "client library" is confusing as it is not implementing a client in any way. (Technically, it implements a server, of which the Prometheus "server" is the client… 🤯) Even if we accept that "Prometheus client library" just means "a library to do something that has to do with Prometheus", the title "client library" still doesn't tell us what the library is actually for. (Note that the client_golang repository not only contains an instrumentation library, but also includes an actual client library that helps you to implement clients that talk to the Prometheus HTTP API.)