Add most of the structure! plus several Base Layer docs

docs/base/config/overview.rst

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+
+Overview
+========
+
+The basic idea of course is that the app needs to be configurable, so the
+question is how to go about that.
+
+The short answer is, "We always use config files but can also store config in
+the database where applicable."
+
+The advantage to config files is that they are easier to get started with, but
+also we can restrict access at the file system level, which means we can (more
+safely) store sensitive information in them.
+
+The advantage to storing config in DB, is that one can change config on-the-fly
+using e.g. the web app.  (Whereas changing a config file requires the app to be
+restarted, so it will read the new file contents.)
+
+Generally speaking, certain config which is needed during app startup (e.g. DB
+access credentials and logging config), or which is deemed "sensitive"
+(passwords, API keys etc.), is kept in config files, and the remainder is kept
+in the DB (if there is one).
+
+This behavior is itself configurable, but most typically, the app will not care
+where a given setting is defined (file vs. DB) but if one is defined in both
+places the DB value would win.  But the app can request a config value from
+"file only" and ignore the DB, where that is desirable (e.g. when reading
+settings required for startup).
+
+We are still in the Base Layer docs at this point, so we'll focus on the config
+files first, and come back to the DB near the end.