docs: filesystems: convert xfs-delayed-logging-design.txt to ReST

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

Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Link: https://lore.kernel.org/r/2233c248f12e7b465cd27ee30a86f96eb632946a.1588021877.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 14e298d..0fd90f85 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -117,4 +117,5 @@
    udf
    virtiofs
    vfat
+   xfs-delayed-logging-design
    zonefs