docs: filesystems: convert configfs.txt to ReST

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

Also, as this file is alone on its own dir, and it doesn't
seem too likely that other documents will follow it, let's
move it to the filesystems/ root documentation dir.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/c2424ec2ad4d735751434ff7f52144c44aa02d5a.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/usb/gadget_configfs.rst

@@ -24,7 +24,7 @@ Linux provides a number of functions for gadgets to use.
 Creating a gadget means deciding what configurations there will be
 and which functions each configuration will provide.
 
-Configfs (please see `Documentation/filesystems/configfs/*`) lends itself nicely
+Configfs (please see `Documentation/filesystems/configfs.rst`) lends itself nicely
 for the purpose of telling the kernel about the above mentioned decision.
 This document is about how to do it.
 
@@ -354,7 +354,7 @@ the directories in general can be named at will. A group can have
 a number of its default sub-groups created automatically.
 
 For more information on configfs please see
-`Documentation/filesystems/configfs/*`.
+`Documentation/filesystems/configfs.rst`.
 
 The concepts described above translate to USB gadgets like this: