Automatically update the website's documentation page for new releases

[yorickpeterse]

https://inko-lang.org/documentation/ contains links to the manual, currently linking to the documentation for "main" and for "latest".

When a new release is published, we should somehow update this page such that it also includes a link to the documentation of that specific version, along with a list of past versions (up to 5 per major version, to prevent the list from growing too large).

[yorickpeterse]

In e68e1e0 I adjusted the release template to include a task for updating the documentation page as part of the release work. This removes the need for any (complicated) automation.