MAINTAINERS: erofs: add EROFS webpage

Add a new `W:` field of the EROFS entry points to the documentation
site at <https://erofs.docs.kernel.org

>.

In addition, update the in-tree documentation and Kconfig too.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20231117085329.1624223-1-hsiangkao@linux.alibaba.com