[docs] update adapter READMESs to include astro add

[sarah11918]

Changes

Updates all adapter package READMEs to include the astro add command. (Still keeps the manual instructions as an alternative.)

NOTE: Awaiting conclusion of discussion re: do we suggest installing with the --save-dev flag or not for the manual install instructions.

These docs currently do NOT suggest the flag (but, our current deploy docs do so we're discussing for consistency). This PR does NOT add the flag, so if its decided that we do want the flag shown, then we need to add that here.

Testing

No tests, Docs.

Docs

Only Docs here.
/cc @withastro/maintainers-docs for feedback!

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Latest commit: 62b5275

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[yanthomasdev]

Thanks for this @sarah11918, I suggested a small fix, after this LGTM!