docs: need to explain how to run headless

[eseidel]

Our headless detection is clearly not working well enough yet. We should also just add some docs about specifying CI=1 or whatever is is we need.

e.g. https://discord.com/channels/1030243211995791380/1189097166476673075/1233055119063515157

[eseidel]

We should just add a generic "configuring for CI" page https://docs.shorebird.dev/ci

[eseidel]

Would like a generic CI page which says:

• shorebird is designed to work well in CI
• shorebird tries to detect if it's running in CI.
• If shorebird fails to detect for some reason: 1. file an issue with us. 2. use CI=1
  Maybe gives generic tips as to the steps involved?

[eseidel]

I think the only thing stopping me from writing this is I don't know how to put the page at https://docs.shorebird.dev/ci itself. I guess I could make a https://docs.shorebird.dev/ci/other url or something.