1. Load page in browser, say https://rocm-documentation.readthedocs.io/en/latest/GCN_ISA_Manuals/testdocbook.html
2. Press the F12 key on the keyboard to open the browser's developer pane. (By default the developer pane is usually docked to the bottom of the browser window, so the full width the content pane is visible.)
3. To disable content column width limit:
Click the content selection arrow tool button at the top left corner of the devloper pane.
Click in the white margin around the content column. This should select <div class="wy-nav-content">
Find the ".wy-nav-content" rule in the middle column of the developer pane and hover the mouse pointer over it. It should contain somehing like: