docs: filesystems: convert qnx6.txt to ReST

- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[email protected]>
Link: https://lore.kernel.org/r/ccd22c1e1426ce4cb30ece9a71c39ebb41844762.1581955849.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 671906e..08883a4 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -82,5 +82,6 @@
    orangefs
    overlayfs
    proc
+   qnx6
    virtiofs
    vfat