基本的网格布局，以帮助您熟悉Bootstrap栅格系统中的构建。
Bootstrap网格系统有五种规格，每个设备支持一个系列。每一层都以最小的屏幕尺寸开始，并自动应用于较大的设备，除非被覆盖。
Get three equal-width columns starting at desktops and scaling to large desktops. On mobile devices, tablets and below, the columns will automatically stack.
Get three columns starting at desktops and scaling to large desktops of various widths. Remember, grid columns should add up to twelve for a single horizontal block. More than that, and columns start stacking no matter the viewport.
Get two columns starting at desktops and scaling to large desktops.
No grid classes are necessary for full-width elements.
Per the documentation, nesting is easy—just put a row of columns within an existing column. This gives you two columns starting at desktops and scaling to large desktops, with another two (equal widths) within the larger column.
At mobile device sizes, tablets and down, these columns and their nested columns will stack.
The Bootstrap v4 grid system has five tiers of classes: xs (extra small), sm (small), md (medium), lg (large), and xl (extra large). You can use nearly any combination of these classes to create more dynamic and flexible layouts.
Each tier of classes scales up, meaning if you plan on setting the same widths for xs and sm, you only need to specify xs.
Bootstrap V4.4新增的类-Additional classes added in Bootstrap v4.4 allow containers that are 100% wide until a particular breakpoint.