docs: Update footer with new required copyright notice

[afgambin]

Based on these discussions with web team (mm discussion) and legal (mm discussion), the footer of the CKF docs pages needs to be updated (only those under the Docs tab!).

The change should be that the copyright notice in the footer reads as: 'Copyright © 2024 CC-BY-SA, Canonical Ltd', instead of the current one '© 2024 Canonical Ltd'.

See the Anbox Cloud docs page footer for an example.

In case this is not possible, the agreed proposal is to use the following content:
'Generic website content: © 2024, Canonical Ltd.
Technical documentation content: © 2024 CC-BY-SA, Canonical Ltd.'

Note: filed here, but not sure if there is a Jira form for tracking this kind of work/issues requests.