Document dynamic changesetTemplates and steps.outputs

Created by: mrnugget

This pull request does multiple things:

• documents steps.outputs in campaign spec reference with lots of examples to show how they work
• documents which changesetTemplate fields now allow templating in campaign spec reference
• documents which template variables are available in outputs in campaign spec templating doc
• documents which template variables are available in changesetTemplate in campaign spec templating doc
• change links to Sourcegraph CLI from linking to the github repository to the CLI docs, now that we have them
• switch to using tables to document the available variables in templating fields. That looks like this:

I also decided to do the following:

• remove the Experimental badge next to "Campaign spec templating"
• change the Experimental badge next to fields in campaign spec YAML reference to a Templating badge that links to the templating page. That looks like this:

Why remove the Experimental badge? I think templating is here to stay and features (such as dynamic changeset templates) build on top of it. Having the experimental badge might scare users off, which is bad, since a lot of cool things (e.g.: only run commands over matched files) are possible with templating. And while we will surely update it and/or might make drastic changes to it in the future, that's not different to any other features we currently have.