chore: add Kura 5.6.0 release notes (#5510)

* chore: add Kura 5.6.0 release notes

* First version of release notes

* Removed 5.5.0 related entries

* Added known issues about dhcpd and wifi security

Signed-off-by: pierantoniomerlino <[email protected]>

---------

Signed-off-by: pierantoniomerlino <[email protected]>
Co-authored-by: nicolatimeus <[email protected]>
Co-authored-by: nicolatimeus <[email protected]>
Co-authored-by: pierantoniomerlino <[email protected]>