If you have the latest version of Publii, are using one of the Publii themes from https://marketplace.getpublii.com/themes/ and create your web pages using the WYSIWYG editor, you can use the ‘Table’ button to easily create and edit a data table, set the number of columns, rows, styling, etc. So that’s the easy way.
If you wanted to customize your table precisely, you can choose the ‘Source code’ button, and edit the HTML code, if you’re familiar with that. For example, usually be default, table width is set to 100%, so you can change that to another value, say 85%. You can set column widths too by editing the HTML. Or if you have the code for an entire table you want already defined, you could try copying and pasting that into your source code to replace the entire existing table.
The documentation page is at: The WYSIWYG editor page at: https://getpublii.com/docs/post-editor.html#insertingtables
Some ‘standard’ data table options shown for the ProDocs theme at: https://demo.getpublii.eu/themes/prodocs/table/