Copy-edit the standalone colorbar tutorial

[anntzer]

• The tutorial is mostly about making standalone colorbars, so use that as title.
• Most users will still want the colorbar next to an existing axes, so explain how to do that.
• "Extended colorbar with continuous colorscale" is first about discrete colorbars (extensions are a second point), so adjust the title; also it already needs to introduce BoundaryNorm. Adjust the next example's text accordingly as well, as it now only needs to introduce ListedColormap.
• In that next example, describe the additional kwargs to colorbar() in a bullet list.
• More consistently use fully qualified names the first time a new class is introduced, and unqualified names thereafter.
• Use constrained layout instead of manually adjusting figure geometries.
• Minor additional tweaks.

PR summary

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines