Add an example for using a build profile

[unvermuthet]

I suggest adding this file to hint at the fact that doing this is possible. Disabling unused classes can save a lot of time when compiling godot-cpp.

The included build profile has some example classes enabled. To make use of the feature, a user would discover the file, read the comment in the SConstruct, adjust the profile and uncomment the instruction.

[enetheru]

I like this type of thing, I recommend using a build profile a lot in the discord, so having it front and center for people to experiment with is a "show don't tell" move.

[unvermuthet]

I changed it from being passed by default with no disabled classes to having just a few classes enabled and marking it as an example to be adjusted and included. This was discussed and agreed upon in the GDExtension meeting on 1st April. @dsnopek was against adding a comment to the build_profile.json as a key-value pair, but I think it's important for people who discover the JSON file first. They'd likely be confused and might expect changes they make to have an effect when the file is not actually passed by default.

[dsnopek]

@dsnopek was against adding a comment to the build_profile.json as a key-value pair, but I think it's important for people who discover the JSON file first.

It's not my personal preference but I think it's fine :-)

[Ivorforce]

We should definitely move this info to the docs, but we don't have it there yet. So I'm ok with it as-is.
I'm not sure I'd add the comment there like this, I think having the SConstruct comment is good enough as long as the build profile is disabled by default. But it works for me.

[paddy-exe]

Merged. Thank you all!