chore: include more valid bcp 47 tags and add markdown alerts into docs

[Meng-Heng]

The information is from: https://community.software.sil.org/t/cant-install-keyboard-with-transient-language-despite-not-having-any-transient-languages-installed/10233/11.

This PR is ready for review.

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

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