Invert symlink and content for the README file

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/devel/swh-py-template!27

README.rst

-docs/README.rst
\ No newline at end of file

README.rst

+Software Heritage - Vault
+=========================
+
+User-facing service that allows to retrieve parts of the archive as
+self-contained bundles (e.g., individual releases, entire repository snapshots,
+etc.)
+The creation of a bundle is called "cooking" a bundle.
+
+Architecture
+------------
+
+The vault is made of two main parts:
+
+1. a stateful RPC server called the **backend**
+2. Celery tasks, called **cookers**

docs/README.rst

-Software Heritage - Vault
-=========================
-
-User-facing service that allows to retrieve parts of the archive as
-self-contained bundles (e.g., individual releases, entire repository snapshots,
-etc.)
-The creation of a bundle is called "cooking" a bundle.
-
-Architecture
-------------
-
-The vault is made of two main parts:
-
-1. a stateful RPC server called the **backend**
-2. Celery tasks, called **cookers**

docs/README.rst

+../README.rst
\ No newline at end of file