Ignore juju docs for linkcheck

[samuelallan72]

This url is valid, and the anchor exists: https://juju.is/docs/juju/manage-controllers#heading--bootstrap-a-controller

However, often linkcheck fails on this url with this message:

https://juju.is/docs/juju/manage-controllers#heading--bootstrap-a-controller - Anchor 'heading--bootstrap-a-controller' not found

[samuelallan72]

Uh oh, it's failed on another juju docs heading:

(how-to/different-model: line 20) broken https://juju.is/docs/juju/environment-variables#heading--jujudata - Anchor 'heading--jujudata' not found

I'll update it to ignore all the juju docs anchors.