docs: networking: Convert alias.txt to rst

The kernel documentation is now restructured text. Convert the IP
aliasing documentation and include it in the toplevel kernel
documentation.

- Fix heading adornments.
- Correctly indent code snippets.
- Limit line length to 72 characters inline with kernel documentation
standards.
- Add license identifier.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>