[i18n] Add Bengali Wikivoyage Welcome template in welcome.py
[i18n] Add Bengali Wikivoyage Welcome template in welcome.py

Bug: T196778
Change-Id: I0fd9afd8941ab833cfa05f5e54b3838dd523564c

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core