[docs][infra] Actually integrate sphinxcontrib-towncrier?

[webknjaz]

I noticed that it was added in 18a9589#diff-fa602a8a75dc9dcc92261bac5f533c2a85e34fcceaff63b3a3a81d9acde2fc52R67 as a dependency and remains listed in the docs extra but is not actually integrated in the Sphinx config or the documents.

To complete this, it's necessary to copy and adapt the following example bits:

• https://github.com/aio-libs/yarl/blob/d472b94/docs/conf.py#L29-L39
• https://github.com/aio-libs/yarl/blob/d472b94/docs/conf.py#L72
• https://github.com/aio-libs/yarl/blob/d472b94/docs/conf.py#L446-L451
• https://github.com/aio-libs/yarl/blob/d472b94/docs/changes.rst?plain=1#L7-L15

combined with removing https://github.com/tox-dev/tox/blob/9c59694/.gitignore#L4, https://github.com/tox-dev/tox/blob/9c59694/docs/conf.py#L46C44-L46C58
and https://github.com/tox-dev/tox/blob/9c59694/docs/conf.py#L96-L101.

[gaborbernat]

PR weclome 👍