editorconfig: add rst extension

editorconfig: add rst extension

We have a lot of .rst documentation; use editorconfig rules for those.
This sets the default tab width to 8, which makes indentation consistent
and avoids requiring developers to adjust editor settings manually.

Link: https://lkml.kernel.org/r/20251219-editorconfig-rst-v1-1-58d4fa397664@gmail.com
Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
Cc: Danny Lin <danny@kdrag0n.dev>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Mickael Salaun <mic@digikod.net>
Cc: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>