Improvements to output rendering for VSCode.

VSCode's IPython notebook rendering works slightly differently than JupyterLab and Colab. This commit makes a few changes to improve the way Treescope works in this setting: - Inserts and removes small amounts of padding, to trigger VSCode to re-evaluate whether "cell output" blocks should be hidden. Previously, VSCode would have false positive and false negatives about which blocks are empty due to Treescope's output-stealing behavior. Adding and removing padding forces VSCode to identify that the temporary output cell is now empty and the final output cell now contains content. - Changes the treescope root element to have a white background that extends the full width of the output cell, which can improve appearance when using Treescope to render small values in a dark-mode theme (since Treescope outputs always appear with a white background and dark mode is not yet supported inside a rendering). Also fixes some errors in building documentation.