Add a hook to ensure README files are symlinked in swh-xxx/docs/

The idea is to generalize the fact that all the swh packages should have
a README file in the root directory, but this should be included in the
sphinx documentation, for which the simplest way is to make the file
symlinked in docs/. For several reasons, it's simpler to make this
symlink creation handled by a hook/automation script at doc build time
rather than add the symlink in the git repo of the swh package.