[WIP] Removing .md files as a part of the Dex IdP Documentation migration. (#1810)

* Removing .md files as a part of the Dex IdP Documentation migration.

https://github.com/dexidp/dex/issues/1761
https://github.com/dexidp/website/issues/2


Signed-off-by: Nate Waddington <nwaddington@cncf.io>

* Updating README.md links after .md files removal.

Signed-off-by: Nate Waddington <nwaddington@cncf.io>

* Updating URL as per PR feedback. dexidp.org -> dexidp.io

Signed-off-by: Nate Waddington <nwaddington@cncf.io>

* removing errant ")"

Signed-off-by: Nate Waddington <nwaddington@cncf.io>