Command to say player coords in chat?

You can do that using just tellraw:

/execute as @p run tellraw @a [{"selector":"@s"},{"text":"'s coordinates are X:"},{"nbt":"Pos[0]","entity":"@s"},{"text":" Y:"},{"nbt":"Pos[1]","entity":"@s"},{"text":" Z:"},{"nbt":"Pos[2]","entity":"@s"}]

That is going to display the coordinates but in the form of a decimal number with a lot of digits. If you want to display whole numbers, what you can do is store the values into scoreboard objectives (storing them converts them to integers) and then display the objectives using /tellraw.

This is how you store the coordinates into objectives:

/execute store result score @s x run data get entity @s Pos[0]
/execute store result score @s y run data get entity @s Pos[1]
/execute store result score @s z run data get entity @s Pos[2]

And this is how you display the values:

/execute as @p run tellraw @a [{"selector":"@s"},{"text":"'s coordinates are X:"},{"score":{"name":"@s","objective":"x"}},{"text":" Y:"},{"score":{"name":"@s","objective":"y"}},{"text":" Z:"},{"score":{"name":"@s","objective":"z"}}]