GitLab will display the content of the README file when browsing the
repository. But in case the file is a symlink, it will display the
path pointed by the symlink. There is a 6 year old issue about this:
https://gitlab.com/gitlab-org/gitlab/-/issues/15093

We can workaround the issue by having the content at the root of the
repository and a symlink to this file in the `docs/` directory.

Tested in swh-py-template!27

This fixes the build [1] [2]

[1]
```
10:49:23  Warning, treated as error:
10:49:23  /var/lib/jenkins/workspace/DDEP/tests-on-diff/docs/README.rst:40:hardcoded link 'https://archive.softwareheritage.org/save/' could be replaced by an extlink (try using ':swh_web:`save/`' instead)
```

[2] https://jenkins.softwareheritage.org/view/swh-draft/job/DDEP/job/tests/1741/console

This only includes redirects for user documentation;
external links to the developer documentation would be broken,
but I don't think there are any such link and I don't want to
clutter the file tree unnecessarily.

This moves files around, so it will cause stale or broken links.
A future commit in swh-docs will set up redirects so the old URLs
still point to up-to-date documents.