Add sample docs

[krish2718]

Add sample documentation to main docs.

[krish2718]

@sachinthegreen this works, but the RST files are copied from NCS and rely on NCS custom sphinx extensions, we have below choices:

1. Fork from NCS and remove NCS custom extensions reliance and instead replace those texts manually. This approach has Duplicate maintenance effort.
2. Clone sdk-nrf (it was removed recently from the manifest) and get those extensions work, which needs significant effort as it needs combining two sphinx projects.
3. Remove all RST files from samples and instead point to NCS docs (URLs).

WDYT?