docs: filesystems: convert cramfs.txt to ReST

- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Acked-by: Nicolas Pitre <[email protected]>
Link: https://lore.kernel.org/r/e87b267e71f99974b7bb3fc0a4a08454ff58165e.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <[email protected]>
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index ddd8f7b..8fe848e 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -56,6 +56,7 @@
    bfs
    btrfs
    ceph
+   cramfs
    fuse
    overlayfs
    virtiofs