improve texture pack documentation

[wsor4035]

for context, read https://irc.luanti.org/luanti/2025-07-18#i_6273034 down

we have https://docs.luanti.org/for-players/texture-packs/ and https://docs.luanti.org/for-creators/creating-texture-packs/

the for-creators implies/can be interpreted as needing to copy all textures over, regardless of if your swapping them out for something else, etc. when you only need to put files in your texturepack that your replacing/modifying/etc

additionally 5.1 reference seems to be pointless and probably should be removed

also for-players should just simply link to for-creators to avoid duplicating documentation