no mention of project deprecation in the docs

### Steps to reproduce

Go to the distillery docs at https://hexdocs.pm/distillery/home.html, note that the docs look very much alive and make no mention of the project being archived.

### Description of issue

Some people (like me today, for instance) are likely to waste a lot of time reading the docs, only to find that Distillery isn't working anymore. It might be benefitial to add a short mention about deprecation to the docs, since the docs are the first place where people land after searching.

Open the Distillery documentation at https://hexdocs.pm/distillery/home.html and identify the main landing page or introductory section. Add a short, prominent note that the project is archived or deprecated, then verify that the notice is visible to someone arriving through the documentation homepage.

Written by the indexing model from the issue text.