docs/cables/*.txt seem to not be used in docs, and are not AsciiDoc formatted

[jimklimov]

Key words from these files were not found in the committed nut-website and were not seen in any generated documents so far (e.g. Cables HTML/PDF would be reasonable).

These text files are not AsciiDoc-formatted either, which may be why they were not added to some recipe?.. Although they do seem to be summarily mentioned in docs/Makefile.am as CABLES_DEPS further added to ALL_TXT_SRC as food for dist and spellcheck targets.

[jimklimov]

Additional clues:

• Googling for key words from those text files brought up nothing (strangely not even their sources)
• https://networkupstools.org/cables.html and https://networkupstools.org/docs/cables.pdf on the nut-website are generated from docs/cables.txt (which has a bit of text and includes a lot of images), and does not contain info from these text files