[bibdc] Add documentation for intl_bib_clean.py

[andjc]

Add documentation for intl_bib_clean.py

Discuss:

• Configuration files: default and custom
• CLI arguments/flags as an override to default and custom configuration
• Unicode normalisation with respect to the MARC21 repertoire
• half marks versus ligature tie in Cyrillic romanisation
• diverging interpretations of Lao and Thai romanisation tables (1997 and 2011)
• Use of CESU-8 in Voyager ad corruption of SMP (Supplementary Multilingual Plane), SIP (Supplementary Ideographic Plane), and TIP (Tertiary Ideographic Plane) characters.