Anchors for config options of same name

[ru-fu]

If we have several config options with the same name (for example, one option per storage driver), we cannot link to them in the general documentation (because we don't know which storage driver the user has).
It would be nice if we could link to the config option index instead, where the user can then pick the option that is relevant to them.

That requires adding anchors to the config option index page before every block of options with the same name.