Centralize and elevate configuration documentation

[jmooring]

This would be a major change, so we should not act on this without bep's approval.

Currently the configuration documentation is spread around throughout the site. I propose creating a section for configuration, with a page for each of the configuration tables: root, markup, taxonomies, menus, params, etc.

This section would be located near the bottom, as a reference. It would be nice if docs.json, generated from source, contained the defaults for all of the keys.

Also incorporate intent from:

• Document setting of params, etc from themes #427
• Add services config page #1781
• Document services config #1779
• Document use of configuration environment variables #2154
• Improve config merge section #2186