Reorganize Bokeh's Sphinx extensions?

[tcmetzger]

The amount and complexity of Bokeh's Sphinx extensions keep growing. At some point, we might want to think about reorganizing things a bit.

Most prominently, the bokeh_plot extension is currently used by other projects to include plots in their docs. We could rename this extension (suggested by @bryevdv). Another option would be to move bokeh_plot to its own repo and maintain it as its own package to use in any Sphinx project (maybe just call it "sphinx_bokeh" then).