Fix mostly formatting in various chapters.
Fix some formatting in Configuration.
Stop supporting the `dxrroot` config option.

It makes more sense to remove this option than to document it. Its main purpose is to point to a custom templates directory, and we previously removed support for custom templates. It can also be used, indirectly, to point to a custom plugins dir, but that's also about to be obsolete, with the upcoming new plugin system.
Rewrite and make true the Configuration chapter.
Morph the highlighting docs into general use docs.
Add glossary.
Tweak to getting-started
Factor out the download-and-build steps so we can reuse them in the Development chapter.
Write Development chapter. Merge Tests into it.
Revise and reformat Plugins chapter.
Add some docs about the query language to the Use chapter.
Be a little more specific about uWSGI.
Delete leading dollars signs from shell commands.
Fix some plural refs to glossary entries.
There's nothing useful in Architecture we haven't covered elsewhere.
Remove term "indexer", add index backlink to LD_LIBRARY_PATH.
Empty out the redundant stuff from the readme. Repatriate a few useful bits. Add a screenshot to boost clickthroughs from GitHub.