docs: add blink-cmp-luasnip-choice community source

[becknik]

Hey, this community source should

[saghen]

That's sweet, thanks. Would you be willing to integrate this into the built in luasnip source?

[soifou]

Would you be willing to integrate this into the built in luasnip source?

This feature has already been integrated in #2238

this community source should resolve #743

Not entirely, the builtin snippet source also needs to be adapted.

[saghen]

Ah gotcha, I'll merge this then and add a note once #2238 is merged