Clarify what is the plan with bn, bn-BD, bn-IN, that are now different versions in Mozilla l10n

There used to be a time when there was no bn in Mozilla. There was only bn-BD.

Now, there is bn, bn-BD, bn-IN.

• We only have bn in Transifex and most translators say 'it is almost the same'.
• We haven't got an alpha now because of #29257 (moved) but it will be called bn-BD
• The translators are quite active and our documentation is already translated, they are translating only one language as bn in Transifex.

This ticket is here to take a decision about that.