docs: filesystems: caching/operations.txt: convert it to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Comment out text ToC for html/pdf output;
- Mark literal blocks as such;
- Add it to filesystems/caching/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/97e71cc598a4f61df484ebda3ec06b63530ceb62.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/caching/index.rst

@@ -9,3 +9,4 @@ Filesystem Caching
    fscache
    object
    netfs-api
+   operations