Don't update script documentation when exporting

[KoBeWi]

Makes the editor skip updating script documentation and scene groups when exporting or running headless.

Fixes #107407 (please test)
Probably fixes #105327 (I tested and seems fixed, but the crash is random)

[bruvzg]

Still crashing, but will a bit lower probability (and usually on reexports).

CowData<EditorFileSystemDirectory::FileInfo*>::size() const (/Volumes/Backup/Projects/godot/core/templates/cowdata.h:165)
Vector<EditorFileSystemDirectory::FileInfo*>::size() const (/Volumes/Backup/Projects/godot/core/templates/vector.h:91)
EditorFileSystemDirectory::get_file_count() const (/Volumes/Backup/Projects/godot/editor/editor_file_system.cpp:91)
EditorHelp::_reload_scripts_documentation(EditorFileSystemDirectory*) (/Volumes/Backup/Projects/godot/editor/editor_help.cpp:3200)
EditorHelp::_reload_scripts_documentation(EditorFileSystemDirectory*) (/Volumes/Backup/Projects/godot/editor/editor_help.cpp:3197)
EditorHelp::_regen_script_doc_thread(void*) (/Volumes/Backup/Projects/godot/editor/editor_help.cpp:3187)
Thread::thread_callback(void*) (/Volumes/Backup/Projects/godot/drivers/apple/thread_apple.cpp:55)
_pthread_start (@_pthread_start:37)

[KoBeWi]

Check again.

[akien-mga]

Fixes #107407 (please test)

I confirm that both this PR and #107699 seem to solve that crash.
Which solution should we go for?

[KoBeWi]

From what I see, my PR takes effect earlier, so waiting for thread is not necessary here (I think?). I also block scene group updates.

The other PR is somewhat cleaner, but seems to be only preventing the doc methods from doing something, instead of preventing them from being called at all.

My changes are probably better, but it would be nice if @bruvzg could confirm that.