Add redirects from old docs to new docs

[pedro-psb]

Summary

The old docs publishing was deactivated and its content was moved to the new docs (see #910).

When the moved content is live on the staging-docs, we should:

• publish the last archive version of the old docs. Should contain a migration note on the index.
• add redirect routes to the new docs (preferably mapping individual pages).

We might use https://github.com/mkdocs/mkdocs-redirects.

Examples

• If someone enters https://docs.pulpproject.org/pulp_cli/ no redirects take place, but the index page contains the migration note in the spotlight.
• If someone enters https://docs.pulpproject.org/pulp_cli/installation/, the page is redirected to the corresponding one in https://staging-docs.pulpproject.org/.