doc: connectivity: networking: lwm2m: update links

Update a couple of lwm2m links

- LwM2M: update to the link the current forwards to
- OMA Specworks LwM2M: update to the link to the releases overview page
      was redirected to the LwM2m overview page.
- LwM2M registry: link to OMA registries, where LwM2M registries are listed
      was redirected to the OMA specifications page before.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>