Resolves https://github.com/EbookFoundation/free-programming-books/pull/6429#discussion_r747870575

* Added Bengali Language Support for HOWTO

* Added Bengali Version Hyperlink for HOWTO

* fix comma-colon typos

* Link back to original source and highlight selected language

* Applied requested changes
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>