Add `layout_columns()` to layout articles

[cpsievert]

By maybe prepending to this article? https://rstudio.github.io/bslib/articles/column-layout.html

Note also that we discuss it a bit in https://rstudio.github.io/bslib/articles/dashboards.html (and could link to the article there to learn more)

[gadenbuie]

Following up on #842, here are a few things to bring out in the layout_columns() and layout_column_wrap() docs:

1. layout_columns() starts with reasonable defaults but gives you complete control over the breakpoints for column sizes. Think of this as the modern replacement for fluidRow() and fluidColumn(). When you want control, this is great! But sometimes it's annoying: a column that's the right size at a large breakpoint can easily be too small at a smaller screen size.

2. layout_column_wrap() is great when you primarily want to ensure the elements are at least X px wide. Min width is a key distinction: layout_column_wrap() can lay out columns but guarantee that no column is less than a width.

3. The above points also demonstrate: layout_columns() deals in relative units (part of 12-column grid) and layout_column_wrap() deals with fixed sizes or relative sizes applied equally.