Restructure Doc TOC

* Restructure main page

* Exclude Index files which are not used

* Rework on the documentation flow

* Related/Dependent on https://booting.oniroproject.org/distro/oniro/-/merge_requests/457

* Closes:  https://git.ostc-eu.org/distro/docs/-/issues/80



Signed-off-by: Gururaj Shetty <gururaj.shetty@huawei.com>