Add optimization tips for mobile in Exporting to HTML5

[Calinou]

See https://github.com/godotengine/godot/issues/60914.

cc @mossfrog

[clayjohn]

We should also mention the problem with importing as sRGB in iOS 13 and below Users will need to disable sRGB and "detect 3D" as per https://github.com/godotengine/godot/issues/60914#issuecomment-1127237932

[mhilbrunner]

cc @Faless just in case you have anything non-obvious to add :)

[Calinou]

Fabio seems busy, @Calinou if you add the above, I'm happy with merging as is. Looks good to me!

I still need to mention the sRGB workaround for iOS < 13. Note that disabling Detect 3D will also mean that Mipmaps won't be automatically checked anymore, so it'll have to be explicitly enabled by the user to prevent textures from looking grainy at a distance in 3D.

[mhilbrunner]

@Calinou Friendly bump. :)