Add tooltips to the export buttons

[hakro]

Closes godotengine/godot-proposals#8870

[JUSTCAMH]

Regarding the tooltip for PCK / ZIP, Export all the project files as a PCK or ZIP package would it make sense to add more detail regarding what a PCK / ZIP package is here, or is it expected the user will read docs? When uploading games online, you typically compress the exported result as a .zip file, and I would've guessed that the zip export would do this (but I believe that's not what it does), and that a pck / zip export isn't enough for players to run the game

[hakro]

@JUSTCAMH the text has been enriched.
We can probably also add a paragraph detailing the differences in the documentation, so that the tooltips don't get too cluttered in the editor.

@akien-mga I squashed the commits, unfortunately that made it loose track of you co-authorship :(

[akien-mga]

Thanks!