docs: filesystems: convert sharedsubtree.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 table markups;
- Add it to filesystems/index.rst
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6692b8abc177130e9e53aace94117a2ad076cab5.1588021877.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -33,6 +33,7 @@ algorithms work.
    mount_api
    quota
    seq_file
+   sharedsubtree
 
    automount-support
 

Documentation/filesystems/proc.rst

@@ -1870,7 +1870,7 @@ unbindable        mount is unbindable
 
 For more information on mount propagation see:
 
-  Documentation/filesystems/sharedsubtree.txt
+  Documentation/filesystems/sharedsubtree.rst
 
 
 3.6	/proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm