Docs: explain cluster states

[asaiacai]

Closes #2663

[concretevitamin]

Thanks @asaiacai! What do you think about adding a quick sentence that refers to https://skypilot.readthedocs.io/en/latest/reference/cli.html#sky-status? E.g., "See here for a list of all possible cluster states." Reasons:

• Most new users may not need a detailed explanation right at quickstart
• Explaining erroneous states may be too complex at quickstart

[asaiacai]

@concretevitamin, thanks for the fast reply! Yeah that makes sense. I changed it to just be a cross ref the the sky status section of the CLI docs.

It looks like currently sphinx-click doesn't have a nice way to cross ref click commands/programs according to:

• Cross Referencing Program Objects sphinx-doc/sphinx#880
• Cross-referencing click-contrib/sphinx-click#15
• sphinx-click docs

so I created explicit targets for each of the CLI commands, which probably isn't the most elegant solution, so let me know if you want to change it. It looks like people have been asking for cross referencing commands in sphinx-click but there's been no change for over 6 years...

[concretevitamin]

Thanks for investigating the cross-ref issues in sphinx! LGTM with a small nit.

[concretevitamin]

Thanks @asaiacai!