link to third party extensions in docs

Related: https://github.com/Fizzadar/pyinfra/issues/12

Is your feature request related to a problem? Please describe

I'm a newbie to pyinfra and was looking for third party extensions as the core operations and facts are non-extensive. Specifically I couldn't find a ufw or hcloud (hetzner cloud) operation/fact in core (https://docs.pyinfra.com/en/1.x/operations.html). I skimmed through the doc, website and readme for the place where I could browse third party extensions but didn:'t find a clue there. I did a search on pypi for "pyinfra" and found some of them. https://github.com/Fizzadar/pyinfra/issues/12 lets me assume there isn't any guidance on distributing third party extensions yet. I'm coming straight from ansible by the way.

Describe the solution you'd like

Abstract: discoverability of extensions, encouragement and guidance to create them.

What comes to my mind is:

• a link to https://pypi.org/search/?q=pyinfra- in the docs and on the homepage
• a section https://docs.pyinfra.com/en/1.x/api/facts.html#distributing and https://docs.pyinfra.com/en/1.x/api/operations.html#distributing
• a github repository and a pypi package "pyinfra-extension-sample" or a link to a good working extension that can act as template
• a doc-section "USING PYINFRA -> Third-Party Extensions"

Review the existing operations and facts documentation, especially the distribution sections linked in the issue, along with the homepage and the guidance discussed in issue #12. Done means users can discover third-party pyinfra extensions and find clear guidance for creating or distributing them.