Automatically-updated community metrics

A rather common question is "how many people use preCICE?". We could already visualize some facts on the website.

We currently have, on the [landing page](https://precice.org/):

- Number of stars
- [Number of citations](https://precice.org/#academic-at-heart-100-free-software) (currently broken, see #522)
- [Logowall](https://precice.org/#tried-and-tested-by-a-global-community) (updated manually)

We could also collect some of these facts in the community landing page: https://precice.org/community.html

We could embed:

- Analytics of the various repositories (core, tutorials) from GitHub. For example:
- Number of contributors
- Recent commit activity
- Recent releases
- Number of stars
- Some analytics from [Plausible Analytics](https://plausible.io/precice.org) (analytics of our website)
- Page views per adapter: that would also show the popularity of each adapter.
- Page views per tutorial
- Download counts of the release artifacts. There are some third-party apps that get such data using the GitHub API ([example](https://somsubhra.github.io/github-release-stats/?username=precice&repository=precice)).
- Analytics of the Discourse forum:
- Number of users
- Number of threads

There is probably a lot more, and various implementation ways.