feat: add docs for functions-ktx

[thatfiredev]

I do realize the functions-ktx library hasn't been released yet, so maybe this should only be merged after its release.

[google-oss-bot]

Hi @rosariopfernandes. Thanks for your PR.

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[samtstern]

/ok-to-test

[vkryachko]

Hey @rosariopfernandes , thanks for the PR! You're right, we will be able to merge this once we've made at least one release of functions-ktx(should happen within a month or so). So let's hold on to this till then

[rachelsaunders]

LGTM