Move backup docs into a subfolder on the Pages site

[Siddhartha Kasivajhula]

They are currently in the root directory, and that makes it harder to coordinate with other workflows that may want to use the site, e.g. the existing benchmarks and the new planned ones in qi-benchmarks by @dzoep .

So probably the best thing is:

1. Set the target-folder variable in docs.yml to docs (currently unset and defaults to .)
2. Set clean: true in all workflows and remove clean-exclude since they won't be interfering in the root folder anymore
3. Verify that the backup docs appear at the new link i.e. same as original link + /docs (since GH Pages maps the directory structure directly to web URLs)
4. Update all links to the backup docs (e.g. in the README and in the Wiki)

Once we've done this, it should be easy to add the new benchmarks in a new workflow, deploying to another folder in the root directory, and not worry about accidentally "cleaning" docs and other things.