It should be possible to understand the service documentation pages
without having come from the index page. This is a first step in making
them context-independent.
This commit finishes up the tutorial. It also fixes the individual
service documentation and documents the service-specific / global
analytical settings and context variables.
The general-purpose tag loading code was much too generic and complex,
so this commit hard-codes the supported services. Also adds a tutorial
to the documentation.
This commit is not tested.
