Add @linaria/postcss-linaria to list of compatible custom syntaxes

[kutnickclose]

Which issue, if any, is this issue related to?

None, as it's a documentation update.

Is there anything in the PR that needs further explanation?

No, it's self-explanatory.

[changeset-bot]

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Latest commit: a33441e

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[ybiquitous]

Thank you. Linaria is interesting!