Cleanup ContentAddressMethod to match docs

The old `std::variant` is bad because we aren't adding a new case to
`FileIngestionMethod` so much as we are defining a separate concept ---
store object content addressing rather than file system object content
addressing. As such, it is more correct to just create a fresh
enumeration.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libutil/file-content-address.hh

@@ -117,6 +117,8 @@ enum struct FileIngestionMethod : uint8_t {
     /**
      * Git hashing.
      *
+     * Part of `ExperimentalFeature::GitHashing`.
+     *
      * See `file-system-object/content-address.md#serial-git` in the
      * manual.
      */