Git loader: update README for YAML-based syntax

The Git loader README still reference the old ini-based syntax for configuration file. It should be updated to use the new yaml-based format. Its content is also out-of-date in other ways (configuration path, keys, etc.).

While we are at it, it is nowadays probably pointless to document configuration in README. Rather, we should consistently store configuration information in the python documentation of each module, so that we can cross-reference from one module to another. For instance, instead of (re-)documenting the storage configuration requirements in the Git loader module, we should just point to the corresponding configuration documentation of the storage module.

How about a top-level "configuration" document in each module?

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Migrated from T917 (view on Phabricator)