docs: filesystems: convert ramfs-rootfs-initramfs.txt to ReST

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

Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Link: https://lore.kernel.org/r/89cbcc99a6371f3bff3ea1668fe497e8a15c226b.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 08883a4..b8689d0 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -83,5 +83,6 @@
    overlayfs
    proc
    qnx6
+   ramfs-rootfs-initramfs
    virtiofs
    vfat