Notify superdb-website repo instead of zed-doc-site when docs change

[philrz]

This is related to other PRs:

• Re-format for hugo docs #5489
• https://github.com/brimdata/superdb-website/pull/6
• Stop showing Next and add an announcement bar about SuperDB docs zed-docs-site#87

When we're ready to make SuperDB docs available as part of https://superdb.org/ (i.e., soon), we can merge this PR so that when contents of docs/ change and we want to publish an updated docs site, CI will now notify the superdb-website repo instead of the zed-docs-site repo.

Since the triggering is done via Actions' repository dispatch events, we could merge this change even before there's any Workflow in the superdb-website repo listening for it and reacting to it, i.e., for now the event generated by CI would just go out into the ether, unclaimed. We wouldn't want to be in that state for long since it would mean tip-of-main SuperDB docs updates aren't being published anywhere. But it allows for some wiggle room in the timing of when we merge related PRs.