Document using --template with other branch names

[delucis]

What kind of changes does this PR include?

• New or updated content

Description

• Closes Wrong starter template description (and typo) #2372

• Wrong starter template description (and typo) #2372 revealed that using create astro --template fails if the repository being used does not have a main branch because it will try to copy that branch by default.

• This PR adds a note explaining how to set a specific branch. I made this advice fairly generic, because you might want to use a specific branch for a number of reasons, but if this seems too generic, we could explain further.

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[sarah11918]

Thanks for tackling this Chris, and trying to figure out what I'd prefer here:

• update the command example to include #branch so there's only ONE long command, but mention that branch is only "other than main"
• also have an example of the ASTRO templates themselves, which you can do just with --template blog vs --template withastro/astro#blog as these instructions would suggest (even though that's not the nice, all-purpose pattern).
• instead of a paragraph, show multiple commands depending on what you want to open? An Astro template? Any repository you want? (and if so, maybe the "branch" becomes less important to point out, because it ends up being important for our repository, but we have a nice way of doing it?)

I honestly don't know what I want here! But, non-blocking, and def NWTWWHB! Will approve to get this in if we can't immediately think of something better. But I don't think this is what I'd like long-term as our approach here.