[docs] Fix 404 links (mui#1880)

Author: oliviertassinari

docs/pages/api-docs/data-grid/data-grid.md

@@ -20,7 +20,7 @@ import { DataGrid } from '@material-ui/data-grid';
 | <span class="prop-name">autoHeight</span> | <span class="prop-type">boolean</span> | false | If `true`, the grid height is dynamic and follow the number of rows in the grid. |
 | <span class="prop-name">autoPageSize</span> | <span class="prop-type">boolean</span> | false | If `true`, the pageSize is calculated according to the container size and the max number of rows to avoid rendering a vertical scroll bar. |
 | <span class="prop-name">checkboxSelection</span> | <span class="prop-type">boolean</span> | false | If `true`, the grid get a first column with a checkbox that allows to select rows. |
-| <span class="prop-name">classes</span> | <span class="prop-type">GridClasses</span> |   | Override or extend the styles applied to the component. See [CSS API](/api/data-grid/#css) below for more details. |
+| <span class="prop-name">classes</span> | <span class="prop-type">GridClasses</span> |   | Override or extend the styles applied to the component. See [CSS API](/api/data-grid/data-grid/#css) below for more details. |
 | <span class="prop-name">className</span> | <span class="prop-type">string</span> |   | CSS classname to add on the outer container. |
 | <span class="prop-name">columnBuffer</span> | <span class="prop-type">number</span> | 2 | Number of columns rendered outside the grid viewport. |
 | <span class="prop-name">columnTypes</span> | <span class="prop-type">GridColumnTypesRecord</span> |   | Extend native column types with your new column types. |

docs/pages/api-docs/data-grid/x-grid.md

@@ -21,7 +21,7 @@ import { XGrid } from '@material-ui/x-grid';
 | <span class="prop-name">autoHeight</span> | <span class="prop-type">boolean</span> | false | If `true`, the grid height is dynamic and follow the number of rows in the grid. |
 | <span class="prop-name">autoPageSize</span> | <span class="prop-type">boolean</span> | false | If `true`, the pageSize is calculated according to the container size and the max number of rows to avoid rendering a vertical scroll bar. |
 | <span class="prop-name">checkboxSelection</span> | <span class="prop-type">boolean</span> | false | If `true`, the grid get a first column with a checkbox that allows to select rows. |
-| <span class="prop-name">classes</span> | <span class="prop-type">GridClasses</span> |   | Override or extend the styles applied to the component. See [CSS API](/api/x-grid/#css) below for more details. |
+| <span class="prop-name">classes</span> | <span class="prop-type">GridClasses</span> |   | Override or extend the styles applied to the component. See [CSS API](/api/data-grid/x-grid/#css) below for more details. |
 | <span class="prop-name">className</span> | <span class="prop-type">string</span> |   | CSS classname to add on the outer container. |
 | <span class="prop-name">columnBuffer</span> | <span class="prop-type">number</span> | 2 | Number of columns rendered outside the grid viewport. |
 | <span class="prop-name">columnTypes</span> | <span class="prop-type">GridColumnTypesRecord</span> |   | Extend native column types with your new column types. |