Use images of buttons in new LLDB docs

[hturner]

The new section on debugging with LLDB uses UTF8 icons to represent buttons in the debugging toolbar.

It would be nice to use actual pictures of the buttons here. According to ChatGPT you could do this in markdown

Click the {: .icon } to open the menu.

Then in docs/css/custom.css (which should be included via extra_css in mkdocs.yml):

.icon {
height: 20px;
vertical-align: middle;
}

Pictures of the buttons could be created from the existing screenshots in this chapter.