Remove documentation for Bootstrap 3

[jelhan]

Documentation discusses Bootstrap 3 at some places. As we drop support for Bootstrap 3 in v5, that documentation should be removed or reworded. Searching for BS3 should be a good starting point.

In case you came along arguments or argument values only supported for BS3, please open an issue to remove them. Or even better: Create a dedicated pull request removing that argument.

[emmebravo]

Hi, I'd like to work on this issue :)

[jelhan]

@emmebravo Did you had a chance to work on it?

[emmebravo]

@jelhan hi, yes, but slowly as I got really busy, sorry!