docs: move changelog to docs/en and allow sphinx directives

Now `tox -e docs` will also include the draft changelog for the
next version (locally only).

`CHANGELOG.rst` now only points to the changelog on READTHEDOCS so
sphinx diretives can be used.

Followup to https://github.com/pytest-dev/pytest/pull/6272

.pre-commit-config.yaml

@@ -48,7 +48,7 @@ repos:
     -   id: rst
         name: rst
         entry: rst-lint --encoding utf-8
-        files: ^(CHANGELOG.rst|HOWTORELEASE.rst|README.rst|TIDELIFT.rst|changelog/.*)$
+        files: ^(HOWTORELEASE.rst|README.rst|TIDELIFT.rst)$
         language: python
         additional_dependencies: [pygments, restructuredtext_lint]
     -   id: changelogs-rst