docs: filesystems: convert autofs.txt to reST

Convert autofs.txt to reST.

The following changes abound:

- Introduce reST formatting for headings, lists et al.
- Add an indentation of an 8 space tab wherever suitable, so as
  to maintain consistency.
- Remove indentation of the description of the ioctls which are similar
  to the AUTOFS_IOC ioctls, as it does not come out quite right in HTML.
- Add an entry for autofs in the index.

Signed-off-by: Jaskaran Singh <jaskaransingh7654321@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>