Localizations: Section localization should move

[michbarsinai]

Move to __SECTIONS__.md in the defining file's nodes directory, along with the other node localization files.

This allows for:

• Better re-use of decision graph files, as their localization data can be moved by moving a single directory. Currently, reusing section names requires digging them out of a central sections.md file.
• Better support when changing a .dg file name: enough to change the directory name at the localization, no need to change relevant rows in the sections.md file.