Fix docs links in Doxygen front page.

[JanneKiiskila]

The links pointed still to developer.pelion.com.

Check list

API change(s)

• [x] Not applicable.
• [ ] API is backwards compatible.
• [ ] API documentation is updated.

Example applications updated

• [x] Not applicable.

  Changelog

• [-] CHANGELOG.md updated. (Not applicable).

[JanneKiiskila]

All links confirmed with manual testing.

[JanneKiiskila]

https://github.com/PelionIoT/mbed-edge-examples/pull/24 pulls this to edge-examples.