Do a first pass over campaign docs to prepare for next release

Created by: mrnugget

(I won't get around to finishing https://github.com/sourcegraph/sourcegraph/issues/12784 before I leave for vacation, but I did a first pass)

The docs as they were before this commit reflected a desired state: they were descriptions of what we will implement. Most notably they described:

• the new feature-toggle, that's not yet merged (https://github.com/sourcegraph/sourcegraph/pull/11621)
• ability to interact with the code host on user's behalf (vs. single token). Not yet implemented, see https://github.com/sourcegraph/sourcegraph/issues/10813

I changed those sections to reflect the current state. We can always bring back those descriptions later, once we have the implementation, thanks to the power of git.

I also removed the examples, which are completely outdated and I'd rather remove them now and re-add them later than have users discover and fail to run them.

Then I fixed links, added the YAML to the "track changesets" section, and a few other things.

There are still a ton of TODOs left, most notably the screenshots and actual button names (cc @eseliger, you can fix those once we have the names for the buttons), but I think there's now less danger of users being confused than before and we can always add things.

Edit:

I also explained how to publish all changesets (because we don't support publishing only a subset).