erofs: update the overview of the documentation

erofs: update the overview of the documentation

Update the overview section to better reflect EROFS's design philosophy
as an immutable image filesystem, update the feature highlights with
recent capabilities, and remove outdated items.

The following detailed sections will be revised later since the overview
section is the most visible part of our documentation. Outdated or
ambiguous information could mislead new users and potential adopters.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>