doc: Application <BOARD>_<revision>.overlay description

This commit is a follow-up to #29990 with added board adjustment files
for shields and DTS overlay description.

It adds `boards/<BOARD>_<revision>.overlay` and
`boards/<BOARD>_<revision>.conf` to the list of files containing board
specific adjustments.
It also adds `<BOARD>.conf` to the documentation, as this has always
been supported but not described in shield docs.

It documents the possibility of using
`boards/<BOARD>_<revision>.overlay` files for DTS adjustments to
specific board revisions.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>