chore(books-bn): split bangla books - courses (#6626)

* chore: init bangla book-courses split

Via https://github.com/EbookFoundation/free-programming-books/issues/6591

* chore: Move YouTubes to `courses-bn`

Via EbookFoundation/free-programming-books#6591

* lint: Fix `blank-lines-1-0-2` errors 

99:1-100:121  warning  Incorrect number of blank lines between last section and next heading  blank-lines-1-0-2  remark-lint

* fix: wrong TOC anchor for Java

Completes commit 4fdbe135e6

* feat: HTML comments to fake Markdown equiv

Hack using link references syntax

[//]: # (whatever you want)

[//]: # (
  whatever you want
  multiline
)

* chore: remove commented empty sections

README.md

@@ -80,6 +80,7 @@ Please read [CONTRIBUTING](docs/CONTRIBUTING.md). If you're new to GitHub, [welc
 ### Free Online Courses
 
 + [Arabic / al arabiya / العربية](courses/free-courses-ar.md)
++ [Bengali / বাংলা](courses/free-courses-bn.md)
 + [Bulgarian / български](courses/free-courses-bg.md)
 + [English](courses/free-courses-en.md)
 + [Finnish](courses/free-courses-fi.md)