assembly.rst / .po: invert translation direction

[ThomasWaldmann]

currently, assembly is the only resource translated from German to English - this should be inverted so it is English to German (as we already have for all other resources).

TODO:

• invert the assembly.po file, so msgid is EN, msgstr is DE (also move it to correct locales/ subdirectory)
• invert the assembly.rst, so it has EN text

HINTS:

• maybe the python polib package can be used (supports iteration over po file contents, access to msgid/msgstr, creating new po files)
• i tried to auto-translate the assembly.rst using the mapping from the po file, but that only partly worked: some strings were not replaced correctly - is there an issue with \n vs. blank or with hyphens?
• the source_lang setting of the assembly resource in the .tx/config seems to not work correctly. the transifex docs sound like this setting is per project, not per resource.

Links:

• https://www.transifex.com/thomaswaldmann/multigeiger/docs_build_gettext_assembly/
• https://multigeiger.readthedocs.io/en/latest/assembly.html
• https://multigeiger.readthedocs.io/de/latest/assembly.html