Implement future changelog previews in the docs

[webknjaz]

What do these changes do?

This patch injects towncrier preview into the docs.

Are there changes in behavior for the user?

They'll be able to see what's coming in the next release.

Related issue number

N/A

[patchback]

Backport to 3.8: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 079e9c5 on top of patchback/backports/3.8/079e9c5a7e6fede74be6fabd689b875f2065e084/pr-6055

Backporting merged PR #6055 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.8/079e9c5a7e6fede74be6fabd689b875f2065e084/pr-6055 upstream/3.8

4. Now, cherry-pick PR Implement future changelog previews in the docs #6055 contents into that branch:

   $ git cherry-pick -x 079e9c5a7e6fede74be6fabd689b875f2065e084

   If it'll yell at you with something like fatal: Commit 079e9c5a7e6fede74be6fabd689b875f2065e084 is a merge but no -m option was given., add -m 1 as follows intead:

   $ git cherry-pick -m1 -x 079e9c5a7e6fede74be6fabd689b875f2065e084

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Implement future changelog previews in the docs #6055 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.8/079e9c5a7e6fede74be6fabd689b875f2065e084/pr-6055

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[aio-libs-github-bot]

💔 Backport was not successful

The PR was attempted backported to the following branches:

• ❌ 3.8: Commit could not be cherrypicked due to conflicts